Grotesques + wyches would be mean. Good CC, if you win you auto break them.
I've thought of that (but haven't tried it) and here's the problem:
Right now in 5th edition, the winner of close combat is determined by which side loses the most wounds. So here we run in to our first problem: The enemy can effectively double the amount of wounds they inflict in the assault by targeting their attacks against the Grotesques instead of the Wyches. Actually, it is possibly more then that because they won't half their WS when attacking the Grotesques (unlike against the Wyches due to wych weapons). Since the entire side takes the same modifier (even if one unit took all the wounds and the other none), that is likely a huge amount.
The second problem is with the Wyche themselves: They aren't really lethal, per se. They're fast, get a lot of attacks, and hard to damage (due wych weapons and dodge) but they don't have any unit wide special melee attacks (power weapon, rending, etc) and they're only S3 (1/6th of the time S4). So generally, you aren't going to be dealing a lot of wounds but that's ok because the wych's durability means neither is the opponent. So as long as we deal at least 1 more wound then the opponent does, we still win. However, this is the crux of the matter: The enemy is going to be dealing more wounds then they would normally would against wyches due to the grotesques.
There is one last problem with Grotesques and if it weren't for this there might not have been a problem: Their initiative. Since they're slower then MEQs, that means the MEQ can likely kill off a number of them before they get a chance to swing back. If they were as fast or faster then MEQs, then they could potentially make up for the wounds they'll inevitably take (since they get as many attacks on a charge as wyches most of the time and are S4 to boot). However since they are slower, they are likely to loose a couple first and since Grotesques are kind of expensive... =\