To run better, seriously, start by losing some weight. I've ran 35-50 mile a week for some time, also eating like a pig, and I just kept running worse and worse and gaining weight, in the end was unable to break 6:20 i the mile and 8:30 pace was the hard pace.. I took a two month break (5-15 mi a week just because it's hard to stop so suddenly), started counting calories and lost some serious weight. After that, I'm flying again :).
It's just not fun to jog at 9 minute/mile pace.
As for shoes, wouldn't worry about them. Best runners I've met (sub 4:30 mile) use Nike and Asics, and I haven't noticed correlation between shoe size and how good a person runs. A much bigger correlation usually appears to be between person's gut and how easy he or she runs :)