Whilst I'm not a T2 owner, I've got a download of the manual.

And the way I read the RAM situation is:

The function of the user-installable RAM is to hold samples uploaded into system memory from storage (be that USB drive or internal hard disc) and used in custom voices.

That's all.

It cannot be used to hold styles, registrations, or anything else that is normally stored in non-volatile internal memory, or a USB stick, or on an internal or external hard drive.

If you don't intend using the sampling capabilities of the Tyros then you could probably get along without either an internal hard disc or installing extra RAM. If you are going to use samples then I would get the biggest amount of RAM and hard disc that is available / compatible / affordable.

If using sampling I would tend to use an internal hard drive rather than an external one as I would expect the performance to be better. If you are loading custom voices that use sampling this could be significant, but hopefully someone out there with a T2 can give a real-world indication of whether this is actually an issue.