At least as reliable as any other 80 cubic inch 300HP engine.
The apex seals are analygous to piston rings. They seal the tips (the apex) of the rotor/housing interface
What can happen when you blow out an apex seal is threefold:
1. The rotor and housing can get scratched up
2. You lose compression and power
3. The debris destroys the turbos
Do you need 3mm apex seals?
I don't know.
To protect the stock seals, keep the revs below 4000 until the engine is warm and don't overrev.
You have to take the engine apart to make this change so that even if you can find the parts cheaply, this will be at least moderately expensive.
I would not do it unless I was having the engine rebuilt anyway.
In that case, I'd throw in a street port.
Are you going to race it?