Generic sidemounts and sliders (no sliders for passenger seat as there is no need to move the seat), and Planted seat base which weight about 8lbs each. I usually weld my brackets but these were fairly inexpensive and bolted right up.
I have yet to drive any measurable distance to rate these seats on comfort, but from various reviews I've read, GT3/GT seats are uncomfortable for long trips. They do force you to hunch your shoulders a bit, and it takes some effort to take a deep breath since your chest doesn't have much room to expand. Did I mention these seats are tight?