What everyone is telling you is true - burying the rail has no real danger and certainly no speed advantage. I take exception with Dr B's statement though; any modern, well designed and properly built boat shoud be able to handle the stresses of extreme heel and weather helm without problem. For example, the rig can generally support the weight of the boat (it HAS to, if you think about a knock-down). For example, there are documented cases of boats that have been caught by the mast in lift bridges that sustained only superficial damage. Most modern spade rudders are built very strongly, since they are supported only at one end. A keel or skeg hung rudder will be stronger still. Only abuse such as grounding or hitting a submerged object are likely to damage a well built modern rudder.
Jimjazzda - Point I was making is that by "burying-the-rail", the stresses on the rig and a much higher than when the boat is well balanced. You are right that "properly" built boats should be able to handle the forces on the rig and rudder, but over time things corrode and fatigue and then that is where the rudder post snaps or the front stay or furler shackle breaks, etc.
I was in a race last year on a J105 and the fitting that held on the one of the main sheet blocks while we were heeled over in 25 kt winds snapped and the boom went flying with such force that the skipper almost lost control. Another time I was on Frers 36 in 25-30 kt winds and the clew of the main ripped and that was a mess. Another time on a IMX-40 and we were so overpowered with a 150% genny that the main and boom just bounced around for the entire race on the upwind legs, because any attempt to trim it yielded more heel than the 25 deg we already were. Stuff usually breaks in wind conditions when your "burying-the-rail".
Folks that race typically won't reef a sail if the wind picks up in a race, which means they will finish the race overpowered, something will break, or they will get by wilth inefficient sail trim (main eased/leach flapping, etc.,) Once the race is over, they may put in a reef or throw on smaller headsail, but sometimes that is impractical. I asked a J105 skipper once leaving the dock, knowing the winds were going to be in the 20-25kt range during the race, if we were going to reef the main to make it easier to handle as I was in charge of the working the main. He said, I don't think we have ever reefed the main. Pretty stressful sail for me as I was constantly fighting the main to maintain helm for the skipper.