I haven't set up a test myself, but have read reports/posts from several others. Generally, they're saying a pound increase in bow weight increases speed about 1 1/2-2 fps, all other things being equal. Or, looked at the other way, a 5 fps increase in arrow speed requires about a 2 1/2# increase in bow weight. I'm sure there are diminishing returns as bow weight gets very high.
The difference between a set back bow and a straight bow is about the same as the difference between a straight and string follow bow, again, all other things being equal, which they usually aren't. That would mean you would need to increase your bow weight 4-5# to get the same performance out of a string follow long bow as your longbow with setback limbs, all other things being equal, i.e., design, length, limb material, etc.
On the other hand, you could get a string follow bow in the weight you like shooting now and just accept that it's going to be a few fps slower. Not a big deal at hunting ranges. Good luck.