[pair of jacks]

Hello all- I'm thinking about repowering at the end of the season- having some issues with my Optimax 200 with 650 hrs. on it. Nothing serious, but getting the itch. I'm considering going 4-stroke Yamaha but due to weight considerations I'm told I would have to downsize to a 150 Yami at 475 lbs. vs the 200 Yami at almost 600 lbs.- my current Opti is 500 lbs. and the boat (a 2000 SC 21) is already ass-heavy so adding another 100 lbs. is not something I want to do- my question is will this boat be underpowered with the 150? Opinions??