Upgrading starter wire?

It certainly shouldn't hurt anything, but it is unlikely to help either as the wires to your starter should be appropriate anyway.

If your wiring is old then you may see an improvement, otherwise I would suggest leaving it alone.

It won't hurt anything providing that you have a solidly crimped connector on both ends. As a side note the long crank time is unrelated to the present cable. A new question addressing this issue may get you some helpful responses

with so many accessories being fitted to most fourbys power demands are higher.Lets assume you are upgrading from a 55amp alternator to 110amp.Upgrading your wiring is not only important it is critical.Electricity causes heat and if cable can't handle load placed upon it the cable can melt.To calculate voltage drop

length x current x 0.017

   1000

volts=voltage drop length=total length of wire in metres current=amps through wire area=copper wire thickness in millimetres squared-most car makers use minimum allowable wire thickness.Huge cost savings when multiplied by millions of cars made

THE SIMPLE ANSWER Keep in mind when increasing wire size you may be increasing resistance which means less voltage. So increasing wire size does not necessarily mean it helps get more juice. A volt ohm meter set to ohms could answer quickly the question of resistance in any piece of wire. It should be noted that as wire ages with use it will increase its resistance causing it to become a poor conductor of voltage. Try using same size wire that is new may be your best option. Good tight clean connections are best if your looking to resolve a problem.

6AWG seems awful thin for a starter wire, I can see that being used for headlights, but a starter typically needs much fatter wire.

If the 6 is the wire to the starter solenoid's coil, then that's plenty fat enough, and I can't see increasing it helping, your description seems to suggest that it's going its job correctly.

Hard staring could be a bad engine earth or a bad cable from the battery to the starter.