With my approved installment agreement with the IRS, I'll pay an additional $300. Can I just pay in full before the filing deadline and void the installment plan?

In a perfect world, the IRS would get your payment and cancel the agreement.  That, unfortunately, does not always happen.  To make sure they do not  take your payment and then still do the installment, you need to cancel the installment agreement 

If the installment agreement has been processed and approved, you probably won't get a refund of the application fee. The confirmation letter you receive will have a contact phone number for matters related to the IAP (installment agreement plan). If there aren't instructions (in the letter) on how to pay early, you should call that number and ask for them.

If the installment agreement has not been processed and approved, you should first call the IRS to see if you can "stop" it and avoid the application fee. 

Requests to modify or terminate an installment agreement. After an installment agreement is approved, you may submit a request to modify or terminate an installment agreement. You may modify your payment amount or due date by going to IRS.gov/opa. 

You also may call 1-800-829-1040 to modify or terminate your agreement. There is an $89 fee to modify the installment agreement. Low income taxpayers pay a $43 fee to modify or reinstate an agreement