Found a T-Mobile Support page that answers that question: https://support.t-mobile.com/docs/DOC-24291#subhead2
Also, https://support.t-mobile.com/docs/DOC-24292#question5
So you still get the data compression (which will theoretically make your data allowance go 3x further) but you still get charged for the data used.
Other Pay in Advance
(Prepaid) plans
You get a 30% subscription discount per line for 12 months through Sling.
Important: All video streaming DOES count against your high-speed data bucket.
Also, https://support.t-mobile.com/docs/DOC-24292#question5
I have a prepaid plan. Can I use Binge On?
Yes. Binge On is included with Pay in Advance (Prepaid) plans. And even though video streaming does count against your high-speed data bucket, you can still benefit from getting a 30% subscription discount from Sling. Read about the Binge On perks depending on your data plan.
So you still get the data compression (which will theoretically make your data allowance go 3x further) but you still get charged for the data used.