Hi Winginman,
I have had Dr Pulley sliders in my X500 for about 6k miles now. I highly recommend the use of a dry lubricant like graphite when you install them.
As to mpg, mine took a hit of about 2 or 3 mpg at first, then when I adjusted to the quicker, smoother take off and quit rolling the throttle so much, my mileage came back. The dry lube gives a really smooth take off, especially at slow speeds like in a parking lot.
And highway, that's where I noticed the biggest change. Rpm went up a bit, but I think it's because the engine is not working as hard. Accelerating from 55 to 65 makes the rpm shoot up, but then like another gear, when speed is reached, rpms go down some. At speeds, the engine sounds like it's running easier.