Well, you've asked a lot of questions here, so I'll answer those that I
can.

Tea as a dietary enhancement? Probably not, since the majority of the
chemicals which are supposed to be beneficial are released only by
steeping the tea in very hot water. I doubt that stomach acid or
anything else in the digestive system would work quite as well, so
someone taking tea leaves probably wouldn't get much effect, if any. If
you drink tea, I don't think it qualifies as a dietary suppliment,
since it doesn't really contain that much solid matter anyway. I think
the primary effectiveness of tea as a health suppliment would come from
a placebo effect, rather than anything actually in the leaves
themselves.

I wouldn't associate tea with sport, simply because hot tea doesn't
sound very nice if you're hot and dripping sweat from running around
for a while. Maybe iced tea would be drunk by an athlete, but more
likely for the effects of the sugar and caffiene. And as you mentioned,
tea is a diuretic as well, so that might discourage athletes in some
sports from drinking it. Personally I don't see why caffiene would be
banned by an anti-doping agency, since it would take a lot of caffiene
to cause any major effect, and than it would lead to a major crash,
causing worse performance later.

Is the use of caffiene ethical? I think that caffiene has such minor
effects, and wouldn't help an athlete much other than being a placebo,
so it comes with the same ethical questions that could be considered if
an athlete used a good luck charm, or had religious beliefs which
concerned thier athletic performance.

Personally I just drink tea because I like the taste, and find it to be
a very pleasant drink. I have actually drank it before participating in
sports or work myself, but usually to keep me awake, and generally not
in the hotest parts of the day. I can't say I've ever noticed any
physical benefits, only extra stimulation of my nervous system.