Actually we did all of the Texas parks over Memorial Day a couple of years ago.
Not bad at all!
We did Schlitterbahn Saturday evening and Sunday Morning of Memorial Day weekend and it was very manageable. Like you said, you have to do Master Blaster first as it gets insane lines!
As for the 'to go now, or wait' question. It's really up to you and what you're into. You'll be missing the 'old school' area of the park. Natural river water (colder), older slides, but some stuff you don't see very often anymore. The two new sections that will be open have about as much stuff as an average sized waterpark. (Family Raft ride, two tube slides, master blaster, weird wave/lazy river, surfing attraction, lazy river, kids play area, family blaster, dragon blaster)
Let me know if I can help with anything else!