Depends on the car, what power you want, and what you plan to do with that power.
If you have an early classic, then yes it helps.
If you have a late classic it helps less.
If you have a newage it makes little difference.
300bhp - not much difference.
400bhp - makes more difference.
500bhp - probably essential.
Road only - limited use.
Racing - much more useful.
M
(02 STI, 390bhp, still original top mount).