If you are an electical genius, you could potentially have lots of warning functions as well as a tacho. I guess one of those looms will be for the warning lamps and just lie disconnected in that application.
It should work, unless it's on some Suz data bus kind of system. I'll need to hand over to to someone who has one to confirm that, but if not, pop the guy a mail ask if there is a pole change switch (likely a hole for a small screwdriver) on the back, if so it'll be a pulse counter & fine.
As for 40 Knots form 60Hp, you'd need a vey light boat for that to happen, V unlikely with a deep V rib. I'd guess somewhere between 30-35 for that setup, not knowing any details of the hull or engines For what it's worth I get around 32 from mine when the engine isn't a pile of bits! (60 Hp, me+ boat about 520Kg)
What you could do if it's speed you're after, as you have the twins, go for smaller diameter bigger pitch props - that way the avaialble engine torque will be converted more to forward motion, as you have effectively double the prop blade area between the two, so can afford to loose a bit of diameter and stil lnot "loose grip". Problem might be finding a matched set to borrow to test.