They've added variable valve timing to all the Impreza's and yes with the 2.5 liter engine.
The N/A sedan puts down about 170 HP and the WRX's put out 230 HP with 235 ft. lbs. All Impreza's get a new front bumber. And the STI gets a bumper all it's own.

The WRX TR (Tuner Ready) is basically stripped down in that it doesn't come with the amenities n' sht. And yeah it's a little bit cheaper. It's by far the lightest Impreza and with WRX power it's a huge step over the N/A car. But I don't really see why they would market it as the tuner's car because the STI is by far the most performance oriented of the line-up.