But the percentage is moving 4 times faster than when I only had 16 solars.
Yes, because the limit is not enforced. Nothing keeps the mass fab from accepting any number of EU. It is special in the way that it doesn't have an EU usage rating, it uses anything and everything it gets. However, IF machines explosions WERE enabled, which they are currently not, but IF they were, then the mass fab would explode on receiving more than 8192 EU/t. No matter how many transformer upgrades you put in there. No machine can survive more than 8192 EU/t input (not usage, input) IF machine explosions are enabled.