@Dr John,
Yep a changing baudrate communication would certainly be slower than a fixed speed, but calculating the values take micro-seconds, no FP math involved.
I did the test with 2 Arduinos - UNO + DUemillanove - so one with a crystal and one with resonator (?) and used for both SW serial (you could have seen this in the code 
I did not try a HW serial against the SW serial yet although I did test it with a (19200) HW serial LCD - see earlier post.
This analysis is not final yet as I expect the formulas can be improved a bit for the higher speeds. This can be done by non-linear polynomes at the cost of extra footprint or maybe by slighty tuning the constants in the formulas. Need some time to test (a lot more)