I can only comment on TBI vs quadrajet carb, but the mileage difference for a heavy truck is not likely to be very big. You probably won't get worse mileage, but I wouldn't be surprised if you got the same or only slightly better.
I think the gearing, tiresize, weight, and motor size are far more important in affecting the mpg. For example, I could easily get 15mpg in my k5 when it had 3.08 axles, a 327 small block, and 33 inch tires on 1/2 ton axles using a quadrajet carb.
When I swapped in a rebuilt 350 motor, put a TBI system on it, went to 35" tires, 4.10 gears, and an overdrive trans, I can not get above 11 mpg. I added lots of weight with the axles and tires and I think that killed any improvements caused by the fuel injection or the overdrive trans.
If I had to guess on a suburban you are at least as heavy as mine is now so I wouldn't expect to get much better than 10-11 either.
