Can you enter a jibe at any speed ?
Or you you need to slow down a bit first ?
Assuming good flat water conditions ..
Any examples in entering jibes well above 30 knots ?
At my beginner stage 28 knots is max entry but successful exit is not assured yet ( to get mere 21,3 alpha at the end 
) .
So What was your highest entry speed into successful jibe ? ?
in short .. how fast you can go before entering agresive jbe ?
This is just my opinion and experience...
I've always considered any alpha >20 to be pretty good so you shouldn't frown too much. 
As for how fast you can enter a gybe... I would say as fast as you can go, but the technique completely changes when you're going 30+. With the effect of apparent wind during a gybe at higher speeds I find dropping the sail and oversheeting helps de-power the rig and keep things a lot more controlled and stable. If you look at the fast gybers many of them are doing a laydown gybe rather than the traditional carve gybe... there's just way too much pull on the sail when fully powered up making it a very fine line between going over the front or bouncing all over the place through the turn.
The apparent wind also affects timing of the rig flip and foot change... often my foot change on a fast gybe in flat water is strap-to-strap, meaning the feet are the last thing to happen, while the rig flip seems to happen sooner (but maybe that's because I'm already baring away a lot)
Anyway, that's just my observation. I'm certainly no expert and still trying to figure it all out. Someone else will probably be able to offer better insight into high-speed gybes.
Speed that you can initiate a good Gybe also depends a lot on the board you are on in my own experience.
Generally speaking, narrower boards (WP and tail width) will turn in better at speeds over 30 knots, but often those boards will slow down more during the turn as well.
My IS87 gets hard to sink the rail at more than about 32 knots, but my CA40 will easily turn in at 40 knots on flat water. 
One of the most important keys to a fast Alpha is maintaining speed in the turn and getting back to top speed fast on exit.
