I don't shoot HP silhouete (interested in it though) but as the others mentioned, the problem with 25', 27's, etc for match use is bullet selection. Lots of good hunting bullets but few match bullets. If for example something like a .257 130gr MK was available, a 25-08 might be the hot silhouette round. But since bullets like that aren't available, throats aren't long enough to use them and vice-versa. The 6.5's lucked out because they were made with long heavy bullets from the outset so they're still a natural fit for match shooting 100yrs later.

Flat trajectory has no value for silhouette. You're shooting at known distances, you had sighters, you're going to dial the knobs for each animal.
An old friend shot a 7 T/CU (rifle) He lost the occasional ram, but didn't get twitchy toward the end of a match. Adequate bullet weight plus a light powder charge plus a rifle close to the weight limit yields a flinch free afternoon.