Hi Crazy Chemical,
Sorry for the delayed reply, but I had to make a quick trip (1,909 miles in 3 days) to Florida to pick up 4 boards for my 2008 Starboard demo fleet.
The 8.8 m2 Aerotech VMG is a very powerful sail.
It should be OK on the GO 139, and a slightly larger fin will help you to get planing eariler.
A 6 cm increase in fin span is pretty normal and the 52 should be about right.
But, how much you "pressure" the fin will have alot to do with how the whole package "balances".
If you are pretty "light" on the fin, and are not trying to chase the Formula guys upwind, the 48 cm could be OK.
On the other hand, the 52 (or even up to around 56-58 cm) will certainly help you get going sooner.
There will be a limit to how fast you can go (control issues like "tailwalking" here) with the larger fins, but normally with a sail size as large as 8.8 m2 this is not normally an issue.
A 7.5 m2 with really good top end is where you would begin to see tailwalking in about 18 knots.
Hope this helps,