I'm down to a 150 main jet now and it's just beginning to feel clean at almost full throttle. I suspect a 148 main is going to improve it further. Acceleration in 2nd. 3rd and 4th is amazing, however it seems to get stuck at ~7000 in 4th. I don't think it's over geared, so I can only assume that the exhaust has become the bottleneck, which is presumably why the bigger main jets were bogging it down. SCK have said that my original BigBox (purchased in late 2013) is somewhere between the current BB Touring and Sport, and mine is likely clogged to some extent by now.
The following 'similar' setup uses a 148 main with an S&S pipe (presume a Newline expansion), so I won't be the only one below 150:
http://lfs.alexander-hepp.de/body/singlesetup.php?id=1177&language=0
Marco from Pipe Design has suggested the S-Box 220 III Plus and I'm considering this alongside the BigBox Sport. The release of the SIP/Nordspeed Viper may just be an urban myth. As said previously, a box is my preference, in conjunction with the spare wheel and wide rear tyre, and the S-Box looks better quality on paper. It's derived from a genuine Piaggio exhaust, so wins on construction, but performance-wise, the BB Sport is very well regarded.
Gearing:
Stock PX125 box
68T primary
22T clutch
120/70-11 rear tyre
Trying my best to avoid a big money expansion chamber, but maybe resistance is futile.