FWIW -- my 4800 packs have more runtime than those off-brand 6000 packs.
At our indoor track Orion 4800's don't even come close to the run time of my MaxAmps 6000. 33 minutes with 4300 foam TC... Orions are getting a little over 20 mins. I'm not knocking the 4800's but what you stated is just not what I have seen. As far as punch I think a good set of 4200's may have a little more at first, but it is hardly noticeable. I've had my lipo for about 6 months and it feels as good now as it did when I got it. Could anyone say that about a set of 4200's?
In your case I'd just be carefull of amp draw running a mod motor. The larger the lipo generally means it will handle more amp draw. A 20C MaxAmps 6000 will handle 120 amps constant (calculated off the capacity). You don't want to stress out a lipo and overheat it. If you charge correctly and don't stress out the cell I think you'd love a lipo. And you certainly won't need 6 packs!