So after reading this thread, I jumped over the Straight Talk's site (I already use an AT&T Straight Talk sim card), but I noticed they now show that they have AT&T 4G LTE compatible sim cards. I just ordered one for $6.99 with free overnight shipping. It explicitly states that you can keep your 4G LTE speeds with an AT&T 4G LTE compatible phone.
Hopefully it truly does allows for LTE, that would be pretty impressive for an MVNO! I'm actually on hold trying to talk to customer service to confirm, but if I can't get through, I'll report back after I get my new sim activated.