Thanks for chiming in Arct1k.
I think the real problem with the mounts intended for 40mms is not so much the height, as that can be shimmed indeed (was my first plan with the standard Losi mount too), but the 30mm mounting hole distance, instead of 25mm. This means the motor needs to be mounted a bit rotated, causing sqrt(15^2-12.5^2)=8.3 mm of max gearing range to be 'lost' (basically reducing max pinion size with 17 mod 1 teeth).
On a typical buggy I would need at least 48/35 (pinion/spur) gearing (43.5 mm shaft distance with mod 1 gears) to make it useful. Do the RCM mounts for standard 40 mm motors accommodate that you think, given 8.3 mm sliding space is lost with a 56mm motor?
Putting some numbers on it actually made me realize the 8ight platform is not that suitable with its off-center diff, as the required gearing would put the motor partly outside of the car (great for cooling though!)... Thinking an MP9e would be a better candidate now, but open to suggestions for other platforms. Also wondering about a GT2, but think I want something a bit more robust at non-smooth surfaces.
Ah, thought it was a 2 slot slider mount like the 5T one, but just realized the RCM mount for the 8ight is a rotating one, so that means 30 mm mounting hole distance is out of the question anyway, like with the Losi mount.