Here's an earlier thread on this very subject: Just how far can you modify an enfield?

I remember reading about Ellwood Epps doing one-piece stock conversions as well as reaming out for his .303 Epps cartridge, which is a bit like an "Ackley-ised" number; very similar to the "improved" .30/40 Kragicon. Not recommended with max loads in a SMLE, but popular conversions on P-14 actions. Epps also further wildcatted his improved case to 8mm, .338", .35" etc.

An "improved" .303 case will hold a LOT more powder and will match a .308Win.

There is, however, the perennial problem of getting the radically different case to feed from a SMLE / No4 type mag.

A 6.5 - .303/.30-40 Improved would be quite a screamer on a previously "sporterised" spare P-14 action, assuming you got it for peanuts and the barrel and stock at a discount. Or, you could just chamber it for the Dutch / Romanian rimmed version of the famous 6.5 x54 Mannlicher cartridge.

There was also a .303 ICL improved from Canadaicon, again mostly built on P-14s or reworked M-17s and Arisakas.

If you want more grunt when rebuilding a pre-sporterised P-14 , why not just go to 7.62 x 54 Russianicon?