This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
First of all, busmastering will generally not do much with the transfer rates themselves. What it does is relieve the CPU from the chore of shovelling data around, so that it can do something more useful. However if you tend to do only one thing at a time, or you're running a benchmark, the extra CPU time can't be used for anything and busmastering will have no appreciable effect.
Second, you will not see any decrease in CPU usage with the System Monitor in Win95 because it tracks the amount of time spent in the "system idle" loop. However, with a pending I/O operation, the system doesn't spend its spare cycles there, but in the "waiting for I/O to complete" loop.
Third, you need to use DMA to profit from the improved data integrity and enhanced transfer rates of Ultra-ATA.