I added a ninth cell to my 8-cell 4200 Tenergy pack. In short, it ain't worth it.
I didn't measure top end, but there was no apparent gain in performance and it was much less fun to drive. The added weight made it harder to wheelie and jump and handling suffered too much.
I'm new to brushless RC, but it seems you'll do better buying a high quality 6 or 7 cell pack then to add more cells. It just adds too much weight and around 7-cells you seem to hit the law of diminishing returns. Or at least with the cheapo Tenergy packs. In fact, I'm enjoying 7 cells more than 8 cells. The story is probably different with Lipos due to less weight.