I've been doing a lot of overclocked superfx work lately due 3 commissions. I re-did my personal copies with much cleaner mod-work.
I learned that even among the same part number some eproms go faster than others. I find for 8mbit the m27c801 eprom has the most success. The only way to know an eprom speed is to burn a game to it, plug it in, and see how fast it goes without freezing. The eprom is the biggest factor for maximum overclock speed. I have one eprom that only plays a game up to 45 mhz. I have another eprom which is the same part number as the 45 mhz eprom with the same game image burned on the same cart that plays it happily at 53 mhz. I've been burning images to my collection of 8mbit eproms and testing them individually so I know which ones go faster and which ones to avoid. Here's how I make the commission:
After selecting a good speed eprom I solder in a socket to the pcb I'm making the cart out of, plug in the eprom, and wire up a safe speed oscillator. I play through the entire game start to finish because it's very important to make a cart that doesn't freeze or glitch. These carts are thoroughly play-tested to ensure as much as possible that they don't freeze. I'm overclcoking these games to their absolute limit so this is the only method to know for sure.