Yeah, it'll push your tinny around at a pretty good clip. If you get the weight distributed well, and have the trim set on the right hole, it'll almost certainly plane your boat and give you up to about 15-17 mph. Plenty fast for most 12' aluminum boats.
A 10 hp will get you going about 21 mph on most small boats. On my 12.3' aluminum most 7.5 hp motors won't plane it. I have one that will, but it still won't plane two people whereas a 10 hp will.
Depends on whether you want to use your boat now or wait until you have about $600 to buy the 9.9. The 9.9 will be a far superior outboard for most 12' aluminum boats.