I've read all of the ads about gains from replacing a Holley carb body with either a Proform or a Holley HP body. What's everyone's real world experience? Any gains to be had or should I just mill the airhorn off the stock 4779?

With nothing else changed but the main body to a Proform I picked up .18 and 4mph right out of the box. Same jets, squirters, pump setting and all. It was well worth the money.
I did try a back to back with a HP body and the Proform and there wasn't a difference in either feel or ET. It did have a crisper idle sound with the HP body though.

Better off selling the carb and looking for a good used 850. Or if you are dead set, buy a base plate for an 850, making the necessary mod to the idle circuit. You will gain a little flow with the bigger base.

It depends on several things.

What carb do you have now and what main body are you going to?

What engine and what max RPM are you putting this carb on?

It also depends on how good you can get the tuneup after the main body change. If the new main body flows more air it will need more fuel.

I'm in the low buck small block footbrake world. This is a 4779-9 750 DP carb.

i picked up very little with my HP body replacing a 4779 750 dp.

I've used a couple of the Pro-form to make alky carbs...they worked very well after changing the boosters and working the meetering blocks along with the base plate.

I would have to agree with Ten5 on the performance pickup. My car picked up about a tenth also. Seems like I saw an article that the Proform actually flowed about 785-790 cfm, because of not having the square air horn area above the venturis. Even with the square choke horn milled on a std 4779, there is the square above the venturi, that the proform, and Holley Hp dont't have. Just my $.02