Skeet,
I've been using a standard Dye Ultralight 0.688) for the last year and a half with next to no problems. (this is on an X-mag, Timmy and E-blade)
Multi-bore kits will only help with Gas consumption, not accuracy IMO, hell - look at 90% of Pro/NXL teams - they use the largest bore/insert possible (even the SP teams
) and they don't seem to miss many shots do they
Save your money - get a large bore barrel and spend the rest on training and hookers