Just looking for some clarity....
Are you planning on upgrading the motor?(like you want to spend 3K to go faster, or is that including a rebuild?)
If you do some port work on the 915's have decent compression now, and upgrade the cam, you will have a real bruiser, for not a lot of money.
A demon 750 can be a good carb, but it depneds on which demon 750. Speed, mighty, or race. You pick the series to match your cam.
I have had good luck with demon's but some haven't, and even I have questioned thier QC in recent times. I switch my 750 mighty out for a 950 hp bigs, and saw some improvement, but nothing night and day.
Any brand carb will work, as long as it is close on the size, and set up right.