What year is your 8.0 m2 Retro?
I've always used 490 IMCS 27-29 masts in my 8.0/8.5 Retro sails.
On your question:
" If 2 masts are the same length and MCS can they still have different stiffnesses in their top halves."
Yes, there can be a small difference. The overall bend ratio can be the same (measured at the mid point with a 30 Kg weight) but the idividual bends can be somewhat different and still give the same deflection at the mid point.
The head cap issue is a tough one.
The top of the sail really needs to be opened up to get to all load bearing fabrics up in the top.
Sewing a strap on the outside might work, but this is something I'd send back to the loft.
So, your Powerex 75% mast that you use in the 8.0 m2 is a 490 cm IMCS 27-29 or something different?
I wouldn't think a 460 cm IMCS 24-26 would work at all in your 8.0 m2.
Hope this helps,