I love TPA! So much nicer and easier than MCO. Anyway...yes, traffic can be brutal, especially at the time you'll be traveling. I did it, in reverse, on a Sunday night....still plenty of traffic at 2pm!!! Anyway...for an 8pm flight, I would plan on leaving MK no later than 3. That gives you time in the park, and you shouldn't have to worry overly much about getting to TPA in time. That gives you 3 hrs.....should be more than enough. Grab a bite to eat at the airport.