overhead shots (smash,drop,clear) the correct grip

I believe the technical term is "round-the-head" forehand.
If you search for that, you may find more answers.
eg.) http://www.badmintoncentral.com/forums/showthread.php?t=43822

Anyway, I think if you use the regular grip, you may not hit the shuttle squarely, which will cause it to head down and left (for a right-hander).
The further out to your backhand side the shuttle is, the more you should rotate the racket towards a panhandle grip.
Of course, I could be completely wrong - I use a panhandle grip for everything, just because I injured my thumb years ago (not playing badminton) and find other grips (esp. backhand) painful.

If it hurts, that's a pretty big clue that you're doing something wrong, and I would stop doing it immediately.

I don't think you need a bigger swing. The thing with round-the-heads, is that pretty much all the power comes from forearm pronation, unlike regular forehands where the shoulder also contributes a lot aswell.
Personally, I find it helps a huge amount if I swing my non-racket arm across my body as I play the shot - it just seems to keep me more balanced and stable, which helps with both control and power.

 

As a general rule, there isn't a different grip depending on which corner you're in. An overhead forehand hit from the backhand corner uses the same grip as one hit from the forehand corner.

Having said that, it's good to be flexible in your choice of grip. For example: when you are playing a round-the-head forehand at stretch (under pressure), it's often difficult to keep the shuttle from going out at the side. In this situation, a move towards panhandle can help: quite simply, it angles the racket to point into the court more. This may limit your power on smashes, however, especially if the shuttle has gone behind you (so maybe play a drop or clear instead).

It's hard to know whether that's because of your grip, or simply inconsistency in the angle you hit at. With smashes that are in front of the body, the typical problem with the basic grip is that you slice the smash -- meaning that you lose power, and it sounds weaker (not so much of a BANG).

It's also possible, however, that this slicing is causing you to lose control over the steepness of your shots.

This is probably a sign that the grip you're using isn't suitable. You shouldn't be getting any pain.

The pain could also be a sign that you are holding the racket too tightly; the racket handle may be too small for your hand.

You may be over-doing this adjustment. When you first try a technique, it's natural to exaggerate it in order to feel the difference from whatever you normally do. With this technique, however, any exaggeration is fatal! The adjustment must be subtle.

I've found that this particular nuance -- the slight move to panhandle for smashes -- is extremely sensitive to error. If you go slightly wrong, then it may ruin the shot or feel uncomfortable. You may find it helps to take a break from using this "smash grip", and re-establish your confidence in using the basic grip.

When I first tried this "smash grip", it really screwed up my overhead shots. Then I dropped it for quite a while, and only recently remembered to try it again. This time, it's working really well.

Bigger than what?

Overhead shots, and especially smashes, do tend to have bigger swings than many other shots. And it depends on the smash -- a full-power smash will have a bigger swing than a half-smash.

One idea that I find helpful is to concentrate on keeping your swing compact: even when using a relatively large swing, you need to cut out unnecessary flailing or body movement. Keep in tight, keep it sharp, keep it compact.

With round-the-head shots in particular, your swing will necessarily be quite short -- as Sketchy explained, the shoulder movement is much more limited here. The more extreme the round-the-head, the more limited your swing will need to be. "Keep it compact" is especially important for round-the-heads.

 

It's just same like the regular forehand, but make sure that you're able to bend your hand over the head, when I try first one, I get some muscular problem, it's hurt so much...but the overhead has more benefits than backhand hit, it has more power...