F4U-4 is perked. Fastest corsair but slower than a 109-K4, Pony, Dora, and at less than 10K it's also slower than La7, P-47N, P-47M
F4U-4 will also be out climbed by every 109 except the E, every Spitfire from the Mark VIII onward, and a Pony D. It's about equal to Dora.
F4U-4 is nothing special in the gun department.
But it's perked.
perhaps there is a reason other than it's Uberness that determines perk cost. 
You're just comparing it to superior planes stat by stat. And I don't know where you got your numbers. The F4U-4 is faster on the deck than a P-51D, 109K4 and can match La-7 and 190D9.
Why is the Tempest perked? A Bf-109K4 can easily outrun it above 16K, climbs worse than a La-7 and the armament is worse than a mossie's!

See? There will always be a better plane in a specifit stat, as Saxman put it it's the whole package that matters.
The F4U-4 is one of the fastest on the deck and little else can outrun it at altitude, climbs well and can carry a lot of rockets, bombs and fuel. This performance envelope is shared by no other aircraft in the whole planeset, not even the Tempest.