The Fastlane needs a new ROM and new PALs which allow using 16 meg SIMMs. If your Fastlane is working with 40 megs, that implies that the PALs are not the ones which support 16 meg SIMMs because they ONLY support 16 meg and 4 meg. The non-16 meg SIMMs ONLY support 4 meg and 1 meg SIMMs (40 megs would be 8x4 meg and 8x1 meg).
Since I don't know how new 16 meg PALs would handle 1 meg SIMMs, I can't say I know this for sure, but my advice would be to get four 16 meg SIMMs and test them before getting the other twelve.
