Great you already have the 60 :) I think it's the best all around length - the 50 is fun, but you do lose some upwind pointing ability..
The +1 shim seems to really loosen the ride - less drag on the tail = so less stability from the tail - try it first even with your current tail wing - you can just chuck a washer under the front bolt if you don't have a shim - and yes, does move your feet back - so you move your mast forwards - therefore reducing the 'flying length' of your board, making it more maneuverable (less swing weight).
I tend to ride all my boards with the mast max forwards - Looking forwards to some new designs with longer tracks - ie this one below..