http://stud3.tuwien.ac.at/~e0125557/sro/spgain.html
See how much you get with a 9 Gap at your Level.
Then see how much you get with a 7 Gap.
Subtract the second number from the first number. That's how much SP you'll be losing, (or not gaining, depending on how you look at it), if you go down to a 7 gap.
Sorry, I gotta go.

But there are some links around the forums to SP calculators, if you need them as well.
http://www.sro.mmosite.com/x/
Think that's one, from the top of my head.