I'd take the Frank over any Spitfire any day just for WEP alone.. 3 minutes WEP in the Frank, but it only takes 1/2 the time to recharge.. So a minute or so and you're good to go.. Most Spit peelots will obliviously follow you up in a left hand spiral climb.. They won't be able to hang with you (only works up to 10 or 11K), they'll run out of WEP, they'll stall, you roll over and take 'em out.. Very easy, very effective..
It's all about fighting to your strengths.. And there are many things the Ki-84 does better than just about any Spitfire.. The fire rate of the Ho-5 is unmatched(20mm buzzsaw).. Definitely has the roll advantage over any spit except for the Spit Mk 16.. I'm mainly a 38 stick (don't fly much anymore as real life has been getting in the way), but I dig the Frank a lot and fly it for its similarities in handling to the P-38..
On a final note: Use of trim is QUITE effective on the Frank.. I've taken it past 450 trimmed out properly, didn't lose parts, and had marginal control if the ride..
My opinion of course..