FYI, I'm at or below 200lbs during the ride season, I would say I'm experienced on dirt, grew up riding every type of off road toy and I've been racing mx for my 3rd season now.
Im on a 2010 yz250f (four stroke) and I don't need to "upgrade" power wise. I race against 450s all the time, and beat them regularly so you don't "need" more power, and it's not like you will instantly want to upgrade.
i just spent $1500 on upgrades, and not a penny was in power mods, it was all suspension, so no, power isnt everything.