I dont get any more bars than i got before and 3g and 4g speeds are the same...so no difference here and yeah you should be able to update your prl and profile.
I did at first, but it kept connecting to the weaker Sprint signal which never connected to any webpage. I had to get roam control and force it to roam to verizon's 1X data since Sprint's service is piss poor in my home.
I've had mine for about a week, so I've always been (afaik) on the newer release. I seem to get craptastic (under 256kbps, per the speedtest app) 3G speeds. Wifi and 4G (when I can find it) seem to be fine.
ETA: of course, the second I post this, I start getting speeds up around 500 kbps...