| I can't see how a Full Monty BB will ever match a Full Monty 620 as the only real difference in the tuning allowed is the Big Bore kit, which you will never get the best out of with standard Cams, Ports and valves.
Even getting individual smoothbores setup on short inlet stubs is going to be costly just to try and match the 620s standard setup with a PC3 attached and tweaked on a Dyno. Even the cost of a 6-speed gearbox for a BB bike is going to set you back a bit.
I like the mention by Ian of "creative / highly skilled (and ultimately also expensive) technical fiddling" I would love to go down that route but the rule restrictions prohibit it.
I see as you may as well put a fair bit of money intop a 620 and add a few mods or keep it cheap (£2k max) and run a 583 with some basic mods
As for the standard downpipes on a 600SS they are tiny but then I doubt if the head flows enough for it to be a real worry. | 
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