Uberwaltz, 15 IPS is reserved for special LP's only, I record at 7.5 most of the time; especially with brand new tape.
When you record and playback on your machine, even if it's a little bit off, you will still get good results because your record and playback speed are the same. However, if you buy prerecorded tapes, your machine has to be perfect.
As long as you're having fun, what you got sounds good to me.