The Wildfire is the same speed as the OV, as is the Venture and as will be the Chaser.
The only phones that are faster is the Slider at 800mhz, the Triumph and the upcoming One V.
I wouldn't count on them opening up their system, Sprint HATES that, plus VM's MMS system is already convoluted enough as it is, this would mean adding another patch to make it work on other phones.
As for getting better phones,
Some have claimed they are waiting for the throttle to kick in before announcing more so there isn't a big jump in data use (I doubt this). I can say they are holding back some because of the 4G problem. They were hoping to roll out 4G on prepaid in June, but I've heard that they had to push that back to the end of the year. The 4G announcement will bring new phones, I have already seen one phone I suspect Virgin will get (dual core 1.2, 3.7in screen), however it too is another cheap Chinese phone.