1) Bottlenecking.
The above answer stating "not at all" is actually incorrect. The real answer is YES and NO depending on the game and game settings (such as resolution).
A game like Tomb Raider is likely to have little to no CPU bottlenecking at all whereas some shooters can have big bottlenecks at times especially at 1920x1080 but sometimes at higher resolutions too.
Some odd games like AotS too which are very threaded and can utilize more CPU processing power with 4+ cores/threads.
*The QUICK ANSWER though is I wouldn't worry about it too much as typically a GTX1070Ti + i5-6600K @4.4GHz is going to be GPU bound for average FPS.
2) USED?
I've got mixed feelings on this. If you are certain the Warranty is transferable (otherwise forget it) and there's at least a YEAR left on that Warranty, AND the price is 75% or less compared to a real card (including all costs such as tax) then maybe consider it.
*BUT*
At a quick glance it sounds like you can find a GTX1070Ti for roughly 350-400 (pounds?) if this link is comparable to your pricing:
https://uk.pcpartpicker.com/products/video-card/#c=415&sort=price&page=1
*though clicking on some LINKS didn't show the same prices listed in PCPARTPICKER... for rough comparison of PERFORMANCE:
https://www.techpowerup.com/reviews/MSI/GTX_1080_Ti_Gaming_X/30.html
GTX1070 is roughly 30% faster than a GTX1060 6GB.
GTX1070Ti is roughly 45% faster than a GTX1060 6GB.
3) Will prices DROP in the near future?
It's actually all messed up.
<Moderator - Let's leave the politics out of this, OK>]
There are newer NVidia cards coming but we're not clear on when since NVidia has a surplus of some PASCAL (1000 series) GPU's so it may be possible prices will drop in a month or so. Or not. We don't know.