As you may recall I touched on the various nuances of exhaust design and priority of criteria for ones exhaust earlier in this thread.
But this snippet of info may help guide you and Harrison exhausts in designing your exhaust,
Although mikeinva managed to make good power with the 1-3/4" (44.5mm) diameter D+D , my experience with my exhaust is that he was probably at its limit for flow.
The reason I say this is because the underbelly silencers and mini-cone silencers on my exhaust are the straight though perforated tube type baffle, they offer no restriction in exhaust flow. and although my exhaust is very loud they do take the edge off the volume for shorter rides, but for a trip to the TT with my son back in 2015 which meant many hours on motorway and riding around the isle of man, I decided that I'd fit a couple of dB killers in the mini-cones to try and reduce the volume even more.
The result was that 4" long dB killer reduced the noise marginally but knocked about 10-15 seat of the pants hp off at the upper rpm, so I cut down the dB killers half an inch at a time until they were about 1-1/2 long. At 1-1/2" I reckon I was still losing a few hp in the upper rpm but there was virtually no noise reduction.......so since then I've never bothered with dB killers 😏🤣.
What this told me was that 46mm is about as small a diameter as you'd want to use without losing power in the upper rpm (on my system anyway!),
but don't think using tube alot bigger than 46mm is an easy way to more peak power, exhausts need some back pressure to help produce power at lower/mid rpm. So with bigger diameter tube you could end up in a similar situation as the D+D i.e. you might find an extra 1-2hp at peak but have a big dip in your torque curve at low/mid rpm that even tapered headers can't fix.