In about 2011, I asked Paul Bryant (Viking exhausts) to built a Dunstall 2-1-2, but with a few subtle differences.
I asked for the down pipes to tuck in more closely to the down tubes and for the single pipe underneath to run fairly close to the crossover frame tube. Also I asked for the silencers which were in this case peashooters made by Paul, to run from a higher start point than the standard Dunstall and to more or less follow the standard Commando silencer lines/angles.
The pipes definitely changes the power characteristics a bit.
1. Tick-over was very quiet. (Not a power characteristic...I know).
2. At low revs there was a bit less torque.
3. At (IIRC) about 3,500 / 4,000 rpm the exhaust note changed and the bike pulled noticeably much stronger from there to 7,000 rpm and felt as if it wanted to keep revving which had not previously been the case, but I had to show self restraint and change gear.
As I said, this is using Peashooters which were 1"5/8 diameter, the exhaust tubing enlarging 1/8" at each of the Y junctions, and starting from 1" 3/8 on the down pipes.