It still doesn't make sense from a performance standpoint; the FAA went for the lesser-performing option. 60-Series Merlin variants were optimised for low-level operations. The Seafire III was essentially the same airframe (with subsequent naval mods) as the Mk.V/Mk.IX and that demonstrated a leap in performance over the Mk.V, so comparatively the Seafire III could have demonstrated the same, although its performance was better than the Mk.II.
The only clear statement I have come across is in David Brown's book "The Seafire. The Spitfire that went to Sea" where he notes:-
"Adaptation of the two-stage Merlin 60-series engined Spitfires Mark IX or VIII was impractical because of foreseeable structural deficiencies which could not be overcome by strengthening which would further increase the weight of the aircraft."
The Merlin 60 Mk.VIII was trialled with a hook, but this is where I find the timeline for the decision making regarding the Seafire XV confusing.
From reading my various sources, it seems the Admiralty saw the Spitfire Mk.XII and first expressed an interest in a Seafire variant in Dec 1942. On 29 Jan 1943 Supermarine produced a Spec for the Type 471 to meet it (Morgan & Shacklady). Then on 31 August 1943 a formal Air Ministry Spec, N.4/43, was issued to Supermarine for 3 prototype and 3 pre-production only for a Griffon VI powered Seafire variant.
Also under consideration in early 1943 were a hooked Typhoon (not the 1941 folding wing Sea Typhoon proposal) and Tempest. The former came to nothing and the latter had to await further developments.
The first prototype Seafire XV was ready by the end of 1943 and service trials began in early 1944 at the same time as the hooked Mk.VIII trials (see below).
Production orders were placed with Westland in July 1943 for 250 Mk.XV plus another 300 on 8 Jan 1944. But then there was a bunch of contract changes between Mk.III & XV over the next couple of months.
Then in Jan 1944 8 or 9 Mk.VIII airframes (including JG661,662 & 663) were handed over to the Admiralty and modified at Supermarine Chalgrove, being given arrester hooks (JG661 got a sting hook & the other 2 A frame hooks). Those three aircraft then went to 778 squadron at Crail where "a full range of performance and handling trials together with extensive deck landing trials." (Darling "Supermarine Seafire). He then notes that the FAA had no real interest in adapting the Mk.VIII for naval use, beyond the retractable tailwheel it featured!
Morgan & Shacklady are also confusing about Merlin 60 and the Seafire. After noting the Mk.VIII conversions, they continue:-
"The Admiralty had asked for trials of a similarly modified Mk.IX but the Air Ministry was against such a conversion for it was considered that the next generation Seafire would be used solely for the war against Japan, and as large numbers of Spitfire VIIIs were due to, or in the process of, being despatched to the Middle and Far East war zones, these fighters should be modified at the reception MUs and put immediately into service with the Navy."
But no dates for these "negotiations".
Note this was to be done AFTER these aircraft came off the production line. And no indication of just how "modified" the Air Ministry saw them. Were they to get folding wings? So they seem to me to be more akin to the original Seafire Ib conversions in 1942, rather than a fully navalised Seafire IIc/III then in production and the proposed Mk.XV. Maybe this explains Brown's comments about the strengthening.
Another factor to bear in mind that might have influenced the Admiralty was that in 1943 with the Firefly finally entering service powered by the single stage Griffon and Fairey proposing an improved Griffon engined Barracuda Mk.V in June that year, a Griffon engined Seafire might have simplified the logistics, especially on a space limited carrier.
So had the Admiralty made up its mind what it wanted before the Spitfire Mk.VIII offer was made? Or was its decision made because the Air Ministry proposal for the Mk.VIII was less than satisfactory?
The really annoying thing is that authors devote more time to explaining why it was a Seafire XV rather than a Seafire IV than they do to why the Griffon VI was chosen over the Merlin 60!