I had ep3 6th gear fitted for about a month with my 5.3fd, it was awful 😂
If you‘re planning on going for a different FD later on, personally i‘d just sell the 5.1 and put that money towards the new one.
Save doing it twice, and the 5.1 wont be much of a jump, so when you change again it will also seem like a small jump.
I wouldnt even say my 5.3 has made a big difference, just spend more time changing gear 😂