This is what I have done on my homebuild Taipan F16. I've repositioned the side stay 200 mm further aft and I have never experienced any negative effects. Even when sailing without a spinnaker the boom never seems to go out far enough to touch it. The sail may however, up top, but you'll sail with some very large amount of twist when that happens. My sailing style doesn't seem to include this profile and it almost never happens with me. Note, that I do sail a numbers of times without the spinnaker, when I just want to sail around for an hour or so and not do all the rigging work. Just throwing up the mainsail and go like an A-cat works well for the short sailing trips.

Main point of this post is however that in my opinion TaipanFC has a good point. You can place the sidestays further back without any penalty. The benefits in lower stresses and a more tight forestay are significant though. Going from 500 mm to 700 mm is a large modification.

Best of luck building your boat !

Wouter