regarding kick/topspin serve heights:
I think people are getting caught up with the "height." Where the max height
of the serve is on the court and the speed of the serve is important.
Many people can hit slow kickers that bounce head high, but they have so
little speed that the serve reaches head high before the baseline and will
bounce twice before hitting the back fence. I find that if I can get it head high
or higher at the baseline, it makes it harder for people to attack.
My bread and butter serve is the kicker out wide on the ad side so that's
where I'd hit at 30-40. At 40-30, I might try a slice down the middle.
I try not to use it too much, though or else it gets predictable.