I have never seen any documentation to indicate that C3 was better than 97/130 and that comes from British technical intelligence on the fuel tanks in captured or downed German aircraft. Officially it seems to have remained 96/125. In fact it started of at only 93/115 I doubt you'll find any. Where did you hear that claim?
They were struggling to make enough B4 let alone better the allies 100/130 fuel.
1st part is true, second is not.
150PN fuel is actually 110PN/150PN against which the Germans had 97RON/130PN (test figures not official ones). The PN means performance number and is a percentage increase in power possible when running rich. So allied 100/130 gave a 30% boost when rich. Allied 100/130 was in fact 102/130. Hence 150PN could create 16% more power than 130PN. That would get the Me 109K4 to 2.15 ata and certainly above 2.0 ata.
Or DB gets really smart, lowers down the compression ratio of the DB 605, so it can do eye-watering 2.5 ata.
Both the Mk VIII/XIV and the Me 109 had about 95 gallons/400L in the main fuselage tanks. The Spitfire VII/VIII/XIV added about 25 gallons in the wing leading edges and the Me 109 about 21 gallons in a supplementary tail tank. Little difference.
The MK VIII/XIV have had the LE tanks as-is. Rear fuselage fuel tanks were an tried tested thing for the 2-stage Spitfires, lack of the need (=LW defeated, Allies are near Paris, availability of Mustang Thunderbolt for the RAF) and current doctrine means they were never used, like it was the case for the RAF's Merlin Mustangs.
Was the rear tank of the 109 self-sealing, was it ever used as a fuel tank at all?
If the Luftwaffe wanted to do something crazy and of marginal utility instead of adding a 170 gallon slipper tank they could add 200 imp gallons or 900L of fuel in the form of 3 x 66 gallon/300L drop tanks to the Me 109, this in fact was used on reconnaissance versions of the Me 109. They might try something like a small 'dachelbauch' (dachhound belly, the German equivalent of a conformal slipper tank) tank say 150L combined with the wing drop tanks. No problem, but the Luftwaffe felt no pressing need to do so or felt it was of dubious utility. Useful for long standing patrols over an area yet still in clean range of base.
For the bolded part - here you are wrong. LW was never able to achieve air superiority above UK, above war production centers of the USSR (once the frontline stabilized more or less), and above anything that was 200 miles away from it's fighter bases, even above Malta. The 109, short ranged fighter, was the crux of the matter.
Attaching 900 L of fuel in drop tanks on a fighter that has 400-500 liters of internal fuel does not make it a long range fighter (as in Japanese or US terms), since whatever the fuel is in the drop tanks will be 'dropped' once the enemy is around. The Spitfire VIII/XIV with ~850 L of internal fuel and ~400-800 L in a drop tank is a far better proposal, even though some 30-40 imp gals (~120-160 L, or one of two of rear tanks) should be consumed before entering the combat.
Not too big a slipper tank in a fighter might be a good proposal of you fighter has a considerable performance advantage, not so good idea if the enemy can put in the air a decent fighter of his own. Spitifre already have had 30 and 45 gal slipper tanks (not self sealing IIRC), unlike the 109, BTW.
There was never a chance of an intercooled DB605L since adding 200kg of intercooler and radiator didn't make a lot of sense as opposed to adding 200L of MW50 in the case of the size restricted Me 109. The Me 109 had already accepted an engine size change when it went from the Kestrel/Jumo 210 to the DB601 and then DB605. Fitting the Griffon sized Jumo 213 wasn't possible.
There was every chance of the intercooled 605L, as it was chance for the non-intercooled DB 603L to became the intercooled 603LA, along with a whole host of engines that received intercoolers during their development. Your figure of 200 kg for the intercooler is way above board, eg. the cooling system (engine cooling + intercooling) for the P-51D weighted 663 lbs (~300 kg), the intercooler using maybe 1/3rd of that weight. The figure of 100 kg looks more realistic for a water-to-air intercooler, with further weight saving if the intercooler's radiator is located perhaps under the engine, in a bigger fairing so the oil cooler can share it too. Opting for an air-to-air intercooler further can cut the weight. Another option is the heat exchanger, that would require a bit bigger main cooling system.
Agreed that Jumo 213 or something similar would not fit on the small 109, the 209-II (or whatever was the name) received bigger wings, among other, so it can receive a bigger engine.
Since the DB605 consumed a little less fuel than the Griffon and since the Giffon had to operate on rich mixture (about 20% greater consumption) instead of using MW50 as a charge cooler there was likely no difference in range even if the Me 109 was forced to use MW50 as an Anti Detonant. The range corsing posted suggested the range of 460 miles was at a 226 mph cruise for at least part of the mission which is impractical for even escort combat duty in Europe.
This is what the Spitfire XXI was supposed to give, (not just) range-wise:
link. Seafire 47 also carried rear tanks.
Plain drop tanks were the solution.
Many Me 109 developments, such as the in wing canon armament I suspect, were delayed due to the severe production stresses experienced by the Reich at the close of the war. The use of 4 canon seems to have been limited to the Griffon variants, maybe 700 produced during the war?
Plain drop tanks were partial solution. Greater internal fuel, as CoG-neutral as possible, was the real solution, from Zero and Ki 61 to P-47N and Hornet.
Spitfire V already have had 4 cannons as an option, not a very good one because of performance penalty and not enough of gun heating capacity. Later both engine power and heating capacity were much improved, but threat from German bombers decreased the need for such heavy armament for the Spitfire, though it was re-introduced in 1945 with the modified wing.
Small thin wing of the Bf 109 meant a redesign was needed in order to fit the MK 151 or 108, but it was possible.
