http://www.markwilliams.com/reartech.aspx Here it says that the difference is 5% compared to a 12 bolt. I would bet that the 8.8 is similar. The 7.5 and 8.8 were designed in the days of tightening CAFE. Even at 3% I would be givng up 12 Hp and at 5% it would be 20Hp. That is not a huge amount, but what can I replace it with.
The Ford 8-inch, 8.8", and 9inch rears all have their advantages and disavantages.
Dollar-per-stength, the 9-inch is at the top of the peformance heap. MOST people here putting a Mustang or Ford together are likely to not frequent the track. --some may never set tire on a track. The majority will only see street duty, and that's not likey to be everyday.
There are plenty of people racing for money at the tracks that have 9-inch rears ends under their race cars. Even NASCARs run variants of them.
A person that's building up an engine for power on the street isn't likely concerned if the rear they're running is 2-3% less effecient than an 8.8". The difference would probably go completely un-noticed. 8.8" rears aren't the only ones that can be equipped with disc brakes either, since modern discs can be put on the 8- & 9-inch rear too.
Whenever the conversation turns to Ford rear ends, namely, 8" vs. 9" vs. 8.8", people have their own ideas about what is better. There's always the "9-inch is so heavy compared to the 8-inch, and the 8.8" is so much more effecient than the 9-inch, and on and on...
Bottom line is, it's your (and I'm not sying "you" to any one particular person), vehicle and money, so run whatever makes you the happiest.