Yes, we do, when we release binary packages, e.g., debs and rpms.
The difference exists but there is a caveat, unless you specify -O2 or better -O3 flambda is not really jumping in, so just compiling BAP or any other project under flambda will not give you anything. Unfortunately, setting -O{2,3} for bap is not enough is we spend most of the time in our dependencies, e.g., core_kernel which also doesn’t enable optimizations.
This is a quite unfortunate situation as all the hard work done by the flambda developers is not really used. I wish ocamlopt had an environment variable that we can set to enable optimization even if a package is not enabling it.
Speaking of the numbers, I am seeing improvements between 10% and 30% that are mostly coming from significantly reduced pressure on the garbage collector.