I haven't actually measured difference in top speed but I've noticed in second gear it whines a little bit more about going up steeper hills full throttle, that being said first gear has become more usable especially for bush hogging on my farm which was too rough and hilly to do in second gear...
So after a lot of research and gnashing of teeth I finally have 2 in taller tires mounted and operating on my BX23S. I searched high and low for examples of anyone that's done this and could not find anything so figured I would outline what I did. As any BX owner knows the tiny little toy tires...