It IS possible to disable kext signing (a very good idea)
A good idea to disable kext signing ? Please tell me you made a typing mistake.
Disabling kext signing is a stupid idea of the highest proportions.
Kext signing is there as a valuable security barrier.
Yes, I know that TRIM may be largely unnecessary with modern SDDs but I don't want to upgrade and end up with an unbootable machine or an SSD that gets slower and slower!
Well, the machines is definitely bootable, as I've done the upgrade myself, and yes that's running the latest 10.10.3.
If you are really worried about not having TRIM, then perhaps you should just get yourself a 15k rpm spindle drive instead. That should give you a half-decent performance boost.
Alternativley, when I was doing some research, there was a Swiss (I think) manufacturer who were making SSD drives that had apparently TRIM support under kext OS X. They were a bit cagey about how they achieved it, so I didn't proceed any further, but if you want to persist with a TRIM enabled SSD, they might be worth a look. I will update this post if I remember the name.
Update:
angelbird "SSD wrk for Mac" .... but as I said, not a recommendation, just a name I spotted during research. And its an Austrian, not Swiss company.