FWIW, I put some GT40 iron heads on mine, and transferred the stamped 1.6 rockers from my E7s onto the new heads.
Later I decided that I should upgrade them to 1.7 roller rockers, since that is what Ford puts on those heads from the factory on the 93 Cobras.
Anyway, I got them installed, and I noticed no difference except maybe a little bit in the 5k-6k rpm range. I don't have access to a dyno, but just by feel I couldn't really tell any difference.
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'87 GT, still speed-density. K&N, Explorer non-EGR intake & TB, GT40 irons, Crane 1.7 RRs, BBK long-tubers + high-flow cat pipe, Flowmasters. Soon to be twin-turbo'd (I hope).