The 68010P10 works fine in my A500+. I'm not sure if there are any mods you have to do on a regular A500 or not. I THINK it is a direct replacement, even on the A500, but am not 100 percent sure. I didn't have to do anything on my + except change the CPU. As far as speed, like you, I read 5 to 10 percent, but I can't verify that, as even SysInfo 4 shows exactly the same as with the 68000. They both are running at 7mhz, unless you modify the clocking somehow. If you want to see noticeable speed differences, make sure you have 8mb of fast ram. Fast ram is easily expandable on the A500 with sidecars, or with Kipper's board, and you will see a difference. You didn't say how much fast ram you have, so I am unsure, you may already have it maxed out. Good luck, and enjoy the mods, as that is what it is all about.
