Data speed itself has nothing to do with your phone's response time. Your phone can't display data before it is downloaded. That being said, once it is downloaded to your device, how fast and smoothly and seamlessly it puts it in the form you want is up to the phone's hardware and software. This is where these insane specs and project butter shine. I have the Galaxy Nexus and since Jelly Bean it has been really smooth. I can't wait to see what the Nexus 4 has in store.
As I said, I have the unlocked Galaxy Nexus. It runs on T-mobile's HSPA+ network, but limited to HSPA+ 21. The speeds are very very good. While most of my downloads and streaming takes place over wifi networks, I do on occasion stream videos and music (Pandora) on HSPA+ and I have hardly ever noticed
I don't live in NYC, but given that it is one of the
163 cities T-mobile has lit up with HSPA+ 42, you should be good to go.
What plan are you referring to, exactly? The $70 Monthly4G plan allows unlimited minutes and texting, but is limited to 5GB of high speed data. After that, you get dropped to 2G speeds. If you want unlimited 4G with T-mobile, you will have to sign up for a contract - you can get a value plan though, which requires you to bring your own phone, for fairly cheap if you're willing to sign a contract. Alternatively, you can get a Simple Mobile unlimited everything plan for $50 a month.
Personally, I have plenty of wifi access, and I don't use the phone to talk that much (plus I have ways of routing calls to my Google Voice number over the data connection), so I went with the $30 Monthly4G that comes with unlimited texting and 5GB of high speed data, but only 100 minutes.