I think you could probably go with a longer set of Shocks
- I have done it to try for more off road ground clearance.
Maybe an inch longer would be about as far as I would go.
You will need to be careful about making sure you set the drive chain adjustment loose enough. As the swing arm compresses the shock, the chain will get tighter at one point. Longer Shocks
tend to amplify this effect - you do not want the chain to wind up with no slack - it gets tough on parts.
As for the effect on handling on pavement, it will affect weight bias a little. I don't think it would be much for normal riding, but I can't help you there.