>>Could sufficient "still" air be supplied by a smaller air box with positive pressure ?
Positive pressure like a turbo, yes! There is not much pressure to be had by even a very efficient airbox system so going down on its volume is hard to compensate for. That said, will it make a lot of difference? Are you racing? If not, then not much.
>>My starting point sv650 carbed ,has seemingly a very small exhaust cam , -2mm lift ,-24 deg less than intake
Exhaust cams are usually smaller then intakes, just like the valves. The intake has to suck air from the atmosphere which is hard to do and you can only pull 14.7psi of vacuum so if your intake is not efficient your cylinder does not fill fully which has a horrible effect on power. Exhaust gas can be pushed out by the piston so can have a lot more than the intake's 14.7psi max pressure differential to drive its flow. You can lose some power by having a compromised exhaust cam profile but if it is in the interest of making the intake work better then in the end you have a more powerful engine. This pdf
has a lot of technical information but there is a section discussing each phase of the cam profile and how short changing one can help another. If you can find an online PDF copy of 'Empiricism and Simulation in the Design of the High Performance Four-Stroke-Engine' by Blair he goes a lot more into the design tradeoffs of intake and exhaust cam profile design.
>>I am looking to move power curve down +/- 2000 rpm
That's a lot! Why?
>>I have in mind 650 thumper lunge with v twin balance
Gearing can't accomplish this? A lot of times a 650 single is geared for a much lower top speed than a 650 twin would be and that is what accounts for the seemingly better acceleration. Same reason Harleys are great stoplight machines compared to a GSXR1000. The Harley will top out at 120, the GSXR at 180. Changing the gearing so they both top out at 120 (or 180) and the GSXR would win.
Changing from a CV to a non-CV carb would greatly increase your lungability. The CV carb only opens as fast as vacuum allows. There are mods to be don to CV carbs to increase their throttle response and usually involve increasing the vacuum port hole size.
I'd start by playing with the carbs and cams and gearing before you buy stuff and see how much of an improvement you can make.