I have been told over and over again, the biggest HP gain for the dollar is going roller. Roller motors put up the most HP with the least amount of effort. Hence the reason new cars are getting such impressive HP ratings yet lasting 200k miles. Going roller takes a TON of friction off the internals of a motor thus creating more HP.
Not to mention that rollers can have a much more aggressive and efficient profile to open and close the valves a lot faster than a flat tappet cam. The technology of today's grinds are way ahead of those just a decade ago. I think that is were the most gain in HP is from.
