ditto everything Ken said above...
Also, every disc is slightly different, and no two discs (even of the same mold) will fly the same. There's a chance you've got a particularly understable leopard, when combined with the light weight, and hits taken from dx plastic, you might have beat it in well earlier than normal.
I'ld suggest grabbing another leopard dx (or even champ/star) in possibly a different weight to see how that flies for you.
you for sure on the right path and will be pushing that leopard out to 300+, and that valk to 350+ in no time. as for when the time to "step up" to a faster disc, there is no cutoff point, but you'll know. hell, if you're consistently getting the buzz/roc to 300 on the lines you choose, you may be there now.