same test with stepsize 10 gave some more errors (typical run started with baud rate 10, step 10)
start...
BAUD BYTE
70660 D5 FAIL
81950 AD FAIL
88870 AF FAIL
89570 BD FAIL
94410 D5 FAIL
95340 AA FAIL
96590 D5 FAIL
98980 AA FAIL
100750 AB FAIL
103590 BD FAIL
105740 D5 FAIL
110600 AA FAIL
113260 AF FAIL
120200 AA FAIL
...
Up till 70K no failures ( that are 7000 different baudrates tested !)
between 70K and 115K "only" 13 failures (13 fail on 4500 baudrates tested ~~ 1/300
above 120K the failures increased, not shown
Conclusion from the tests, SoftwareSerial "by formula" works very good up to 70.000 and reasonable well up to 115.200
Tweaking the formulas further may improve the test results but for now I'm quite satisfied.
This SoftwareSerial "by formula" allows one to build a communication channel in which the baud rate is constantly altered, making it very difficult to eavesdrop - and yes to get in sync 