If you applied for a payment plan, and have not had a response yet, it is best to wait for the IRS to contact you.  Usually they will send a letter confirming the payment plan and telling you when the first payment will start.  At that point, you can pay in full, and future payments will be stopped.  The risk is that if you pay now, then after the IRS approves the payment plan they will still start taking payments.

Also, the application fee for the payment plan can't be refunded, even if you pay in full the day after applying.  That's an IRS rule, sorry.