I have both a Mach 427 - but the silver one - and a Losi 454.. Don't know about mixing carbs, but generally speaking, the smaller the venturi, the more low-end punch when it comes to performance, and the better run-time(and sometimes the harder to tune..) I used to avoid going to smaller venturis with my buggy/truggy engines, because I wanted all-out power, but I have been recently.. - I like the feel for ex. of a 7mm over a 9mm. With the smaller venturi the engines feel more responsive(and maybe linear...) Whether you'd notice that in an MT I don't know..

I think the silver Mach has a 9mm carb anyway... And I love the silver Machs..

So yeah, I have no idea.. Sorry.