According to AnandTech, the GTX950-2G has the kind of performance and clock speeds you would expect from a normal GTX 950, despite the lowered power. It has 2GB of 6.6Gbps GDDR5, 48 texture units, 32 ROPs and a 128-bit GDDR5 memory interface. The frequencies have not been lowered, however AnandTech points out that lower power cards often have a reduced hard power limit, so are more likely to power-throttle. It has core / boost clock speeds of 1026MHz / 1190MHz, with 1051MHz / 1228MHZ in OC mode.
In addition, this is the first 75W or lower card to feature hardware-accelerated decoding and encoding of H.265 video, and supports HDCP 2.2 content protection over HDMI 2.0. So, this new card will be useful if you're using your PC to watch high quality video. Good for an HTPC, perhaps?
The Asus GTX950-2G is due for release on March 16, but we're still waiting to hear a price.