To answer your question simply, lets say you are 400hp and running a 3" single exit, you wont get anymore hp by running a 4" single exit, you are already flowing enough air on 3" for say the 400hp, as you go up in power of course, you may need to up the diameter to cope with the air flow needed for say 500hp.
Same goes for running a dual and this is where it gets funky, you would think to match a single 3" you would need two 1.5" dual, but that is not the case, you need to work with pye r squared to work out volume. A few gentle bends here and there make little difference, so a decent dual wont affect flow rates or if the same diameter pipe as a single it wont increase the hp as you already making the most of the breathing your engine can supply. I would imagine as above most guys running single are doing so for the weight saving.
To be fair, when i was booking my fi'd zed to be mapped with hdev i asked what of my two exhausts would give better figures, single fujitsubo or dual hks, the answer was it really wont make any or little difference.
I truly doubt you will get on any track day with a single.
https://www.hotrod.com/articles/what-exhaust-pipes-work-best/