One of the great features of the SPG50s is that you're in a higher "fare class" because you've booked a rack rate. Some hotels are more likely to upgrade you from the rack rate than from cheaper rates.
Have you considered joining Ameniti (search the UA forum)? For $500 you can get 50k UA miles (F to Alaska is 40k, and is a good deal from the east coast) and SPG gold. If the SR upgrades you because of the gold, then that $500 will have been $$ well spent.