Your bloke is probably using similar formulas as I am (from A Graham Bell's "Four-Stroke Performance Tuning"). Bore, stroke, and timing of exhaust valve opening are the basic parameters, then you factor in where you want the peak power and torque in the rev range.
For peak power at 7,800 RPM and torque at 7,000 on an 1130cc 120? engine running F1 cam timing, the number crunching tells me I need 38mm OD headers x 560mm long, going into a 50mm OD secondary. After that, it either doesn't matter too much or it's beyond the scope of simple arithmetic to figure it out. The system I bought from you has slightly longer headers than that, so it might move the power down the rev range a bit at the expense of top end. I haven't got as far as the dyno stage yet.
I suspect the uneven exhaust pressure pulses of a 180? might throw a spanner in the mathematical works, so your development work on the dyno is pretty much the only way of solving the puzzle. Must be costing you a heap in dyno runs and exhaust mods though.