Thanks for all the useful information.

I have been to Malaysia before but not needed to use Grab previously, will need it this time as I'll be going a bit more 'off the beaten track' for treks in Penang, Ipoh and outside KL.
My international to domestic transit in KLIA is about 5hrs, so plenty of time to get landside in KLIA T1 and get a SIM card (cheaper than airside) then heading to the MH domestic lounge before the flight up to PEN. All my previous trips to Penang were by train to Butterworth so then an easy taxi from George Town ferry to a hotel, only this time I'm flying in/train out.