Hi there
I think you will be much better off, taking the plane. It not very expensive and quick.

I have been to Malaysia regularly for many years on biz, only for short times though. I have been flying, taking the train, driving, been driven there. I did not think driving was particularly dangerous (well it is a more Asian style of driving and rules are being not taken all that serious). However imho you can probably do without getting lost, being in traffic jams (in and around KL), paying tolls, having to look at the same sort of landscape for hours in the heat or having to drive in a heavy downpour etc etc.

Rather spend the extra day in Penang