If I recall correctly the Kubuntu install doesn't even offer you that choice, it just installs the default kernel which has SMP enabled. So I reckon this won't be an issue. Apart from that, the MDs don't have to use the same kernel as the Core per se, I have updated the kernel (just recompiled with a small patch really) on my core and the diskless MDs are still using the original kernel.
I suppose I could test with a single-CPU core installation and an SMP MD, but then I'd have to reconfigure most of my network and I'm not *that* bored.