you would get some gain i'm sure but really not as notable as if you make a downpipe too.
we kind of went overkill with the DP by re-using the factory lower support bracket - i'm sure this is a little more than it needs, so long as you have all the exhaust hangers further back doing the job properly, and a good flex joint AFTER the DP. i really feel that the DP doesnt need a flex at all, and have subsequently deleted the flex from mine with no hassles - but, there is a lon flex after the DP, as you see.