Dirk is spot on. Many ratings are tweaked and picked for advertisement purposes. The C rating used on many packs can be a "theoretical limit" based strictly on the chemistry of the cell, not on the complete construction. It doesn't hold up to real world testing.

Here's an example:
I have a B64 just like you, but I have a Tekin RSX and 6.5 turn motor paired with a 4600mah 90C pack. I can typically run for 12 minutes at race pace before the low voltage cutoff hits. In 12 minutes, I consume the whole 4600mah and my charger confirms this amount. This means I averaged 23 amps. If I bought a speed control rated for 30 amps continuous, it would overheat after a few laps.

Most people have found that you need speed controls rated around 40 to 60 amps for spec style classes (13.5 and 17.5) to avoid overheating. For mod, ratings of 100-150 are more typical. Best thing to do is look for someone else's experience with a specific product rather than trusting the numbers on the box.