For starters I don't think your spine is as good as you think it is. In most cases if you shoot field tips well and your broad heads don't group you probably have spine issues. Shooting a tad high/low or to one side of the other can often be touched up by adjusting the rest. However they should still group pretty well.

It is true some set ups will favor a particular broad head, sort of like a rifle liking a particular load. But when things are right it really shouldn't matter as long as they are straight and of good quality.

Some things that may help; more FOC, more fletching for better steering and possibly a lower profile head that would fly better.

When I tune for broad heads I personally don't get crazed about them hitting the same spot as my field points do. Most times they are, but if I know my bow is tuned well and they are a few inches away I don't lose sleep over it. I am more concerned with how they fly and group. I can adjust my sights before hunting season to compensate for a few inches. And while practicing with field tips it isn't that big of a deal. I am more concerned with how they group actually.

I would play with the spine of your arrows and see if it gets better.

Keep in mind that fixed blades are bit harder to shoot well. They will show form errors much better than target arrows do. If you are shooting that well with field tips consistantly I think you could get your broad heads a bit better.

This is so hard to help someone with over the internet. It's much easier to do in person so you can see the set up and watch what is going on.

If you have the power to use them I personally don't see anything wrong with mechanicals. I wouldn't use them as an excuse not to tune though. Mechanicals don't perform too well on arrows that don't fly well.

Paul