what's to much compression for a 4.6 SOHC

A high CR is great for a n/a engine.

Basically the higher the CR the higher the BHP.

But it is not always that simple as a/f ratios need to be checked and if you have a too high CR then regular octane fuel may be insufficent.

Also if any low grade/quality fuel slips into your tank it could cause major problems and damage.

Modern ECU's are very clever and can handle poor quality fuel by altering the timing to suit if it detects pinging, not to sure whether the Mustang is clever enough to do this though.

If you have plans for FI then a high CR is not really what you want, a s/c will require a moderate CR and a turbo is best suited to low CR engines.

With the mods you claim to do high octane fuel may be the only safe way, but I don't know for sure.

Perhaphs try speaking to an engine shop who specialise in the mod 4.6's there are a few out there if you do a search on the net.

the head swap will net you 10.4- 10.5 it not going to hurt you just make uyou get a higher octane gas go for it.