I've got 25's on mine (8's in the front and 10's in the rear), and I am very happy with the compromise. I would guess that I might have lost 5% with the taller tires, but it has more than enough for my riding style. For my needs, a gear reduction wouldn't be necessary.My bro-in-law and his wife both have 300's (and he found mine and helped a ton with rebuilding the motor). He is running 32's on his with 54% GR and very mild motor work. He is really into the mudding/swamping and does some extreme stuff.
His wife's 300 is bone stock with a basic engine rebuild (.020" over I believe) and a small lift kit, and her bike has 27" Mud Lites and it doesn't struggle at all.
I'm running 25" Mud Lites on The Green Monster (my 300) and I feel like that is the best size for MY bike and MY riding, as it gives it decent ground clearance but still maintains a low center of gravity and handles the slants and slopes of our trails like a boss.