I originally didn't beleive that 128Kx8 DIP cache chips came in anything faster than 15 ns, but behold, every desirable 486 cache configuration with 10 ns chips are shown below.

With these 10 ns chips, you can have 256 KB double-banked cache, 512 KB double-banked cache, 512 KB single-banked cache, and 1024 KB double-banked cache. The 10 ns cache over 15 ns really just provides peace of mind when running the FSB at 66 MHz. While not essential for 66 MHz operation, they were the same price as the 15 ns chips, so why not?