Spark in NYC has been a disappointment for me. Haven't seen or noticed any improvements at all (I have an LG G2). Typically test as low as 2Mb to as high as 9Mb usually, which is just fine by me. Fast enough to stream stuff without buffering (most times) but it's just not consistent.
Frankly I'll trade some of that theoretical 40Mbps LTE speeds for better LTE coverage and 3G speeds and the ability to talk and surf over 3G and LTE any day. (I know it's not a big deal for lots of folks but i sure enjoyed it when I had the Galaxy S3 and Note 2).
Definitely jealous of your speeds though.
Lack of concurrent voice/data bothers me too. It's a real shame other carriers are heading this direction too as VoIP by carriers won't be common for a while. Can try Google Voice VoIP in the meantime.