What compression on the motor? The less compression the more fuel you need. 550's should be ok for a while. At 18 psi on and 84mm bore and 9.5:1 cr I maxed my 550's out on 60 psi static fuel pressure and a walbro 255 lph low pressure pump at like 18 psi at 7k RPMs. I say go 650's. They're just as easy to tune, you still get the same smooth idle and they're just as costly as the 550's (I think). I guess the real question is are you stopping here or do you plan to go further in the future? If you're not stopping, get the 650's so you don't have to buy more later.