i always thought the speed gains were to be got from running os8.5 due to it being ppc native - no?
over and above ram considerations of course.
The 1400 is limited to 64 megs of RAM and MacOS 8 adds a lot of bloat over 7.6.1 and the minimum requirements are higher, so that offsets any speed gains to be had from using native PPC code. The 603 is no speed demon, either. It was the replacement for the 601 at the bottom of the PPC family. Head over to System 7 today and you'll see many of the members there use 1400's with 7.6.1 and get good use out of them.
http://main.system7today.com/
And here's their speed benchmarks comparing 7.6.1 to 8.1, 8.6 and 9.1.
http://main.system7today.com/articles/til/benchmarks.html