The ratio between a HDD's outer vs inner track is typically 2:1. So placing games on the outer track will yield a performance of 2 vs. an average performance for the entire drive of 1.5. or a 33% improvement disk speed (25% decrease in load times). Max improvement would be 2:1, or a 100% improvement in disk speed (50% decrease in load times).
A SATA III SSD typically offers a 400% (4x) improvement in sequential read/writes (80% decrease in load times), and a 3,000% (30x) improvement in 4k read/writes (97% decrease in load times). If you enable AHCI mode, the 4k read/writes can improve by up to 15,000% to 25,000% (150-250x, 99.3%-99.6% decrease in load times)).
There is just no comparison. Instead of spending time configuring your HDD for short-stroking, you are better off putting that time and effort into getting a part-time job so you can make enough money to buy a SSD. Really. This is one of those purchases which will save you so much wait time over the life of the computer that you are actually worse off pocketing the $60-$90 and continuing to use the HDD.