My 2p worth,
full monty with a blueflame exhaust which has two baffles. all out doesn't feel any different to just having one out (how I use it)
but if I have both in, I really notice, hitting a massive dip in power at about 3000rpm.
I only ever have both in on the way to MOT though, so doesn't really affect me.
This is all seat of pants stuff, no figures or anything to back it up