I do have the book and my opinion lies somewhat between the two camps.
The Book has answered a few questions about the FW 187s that
did exist. It has cleared up questions about the cooling 'systems'.
It has opened up or failed to answer ( at least in the English version) the question of
exactly which DB 601 was fitted and what power level was used for the 390-1mph flight.
From the wording in the book it
may have been a 1350hp version. The plane
may also have been fitted for two machine guns instead of four and the MG 17 installation on the FW 187 was not exactly low drag.
As for the later "paper" versions. I see no real reason to doubt the author that such documents exist. I am fairly certain that somebody could find all manner of "paper" airplanes in allied archives with absolutely wonderful performance. We sure built enough real ones with performance that didn't come up to expectations. And a few that exceeded expectations.
I am sure FW was trying to pitch the FW 187 design (or offshoots) when ever they could. Many allied companies did the same thing.
Grumman Avenger as a single seat attack plane. Just because a company pitches an idea doesn't
mean it was a
good one. And this was actually built/converted.
FW was pitching one of the later "C" version as a night fighter and said it was possible, (they weren't going to try to sell it and claim it was impossible were they? ) but in the book there are only a few references to this. there is NO drawing of a "night fighter nose" there may be a drawing of an armament lay out but NOT one of a radar layout or installation, there is no photo (or mention) of a wooden night fighter cockpit mock-up(much better for seeing if things
fit than flat drawings), so one has to wonder how serious they really were.
BTW pre war or early war "night fighters" simply meant how suitable the plane was for taking off and landing at night, it actually had nothing to do with
finding the enemy aircraft at night. Shielding pilot from exhaust glare, how tricky to land ( landing speed,
THE reason the Gladiator was fitted with flaps, I mean, how slow does a biplane have to land in the day time?) and fitting a few night
aids like a couple of flares to help illuminate the landing field or 'emergency' landing field when the night fighter can't find it's own field.
The book doesn't seem to make a lot of 'claims' ( or at least more than many other books about single aircraft that tend to view their subjects as war winners all on their own) aside from presenting some of this information.
2nd BTW. The book doesn't present the FW engineers as infallible, and judging by the FW 190 program and the FW 154 one has to rather careful just what is being compared. FW 154 prototype may have hit predicted numbers but when fitted with guns and radar aerials speed fell by over 10%. Production FW 190s changed from the FW 190v1 and V2 in how many ways? Curtiss hit a similar problem with the XP-46. First prototype completed (w/o operational equipment) hit predicted performance numbers. 2nd machine with full armament/equipment failed by around 50mph.