Yes you can pay off an IRS installment agreement early if you want too and they'll be no penalty assessed for doing so.
In fact if you do pay off the IRS installment in full before the term is up it's likely that the installment payment plan fee will be waived.
If you do have the ability to pay off your IRS taxes in full after setting up an agreement then you should do so since getting the IRS tax debt off your back is a good thing.