A lot of people are using OEM Golf R springs with Audi TT rear pads. From what I have read it is about a 1.2" drop and the ride is similar to that of the stock Golf R. I just bought some Golf R springs but haven't put them on yet. Will update when I do.
Have you swapped springs yet? I've got R springs like you and planning on changing out struts and shocks too with Golf R shocks/struts to have exact match for the stiffer/lower R springs. I'm of the belief that springs and shocks should be matched as closely as possible.
I'm concerned that just changing out the springs and not the shocks would cause the stock shock to bottom out causing premature failure also the mismatch of stiffer springs with stock socks would affect ride and handling negatively.
Changing out both springs and shocks to Golf R just makes sense to me. It's kind of pricey I know around $500+ for all the needed (shocks/struts bump stops and hardware) but to me worth it in the long run saving the expense of probably having to do the work twice either because of the ride quality or failure of the shocks or both.
BTW, I did find a local dealer that was ok with the lowering option and said it wouldn't affect the warranty just wouldn't cover the replacement springs and shocks if they were to fail which makes sense.