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).
when i use the basic grip i can perform shots correctly, but when i smash sometimes i hit it too steep, and hit the net.
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.
when i add a slight adjustment towards the panhandle, i feel a tension on my thumb and top of my hand and its pretty painful.
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 HAVE been experimenting, but still can't find the comfort zone.
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.
does your arm swing have to be bigger for overhead shots too?
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.
