I've been thinking the exact same thing. When I do Ul'dah(BW/Hala), I seem to get less SP than when I do them over at Grid (9ives/Tree).
Also, Class Rank does make a huge difference. @R35, When getting carried by a 50 ARC 5* lv.40 leves, I would get 8k SP bonus w/GA. Now @47, I can solo 5* at the same speed, but I'll be lucky to get 5.5k w/ GA.
I, too, definitely notice that doing leves at a slightly lower Star level, but faster completion, grants more SP.