i havnt got the measurements, but is a fair bit. you would defo notice it the moment you sat in it.
with regards to the standard seats, i think it would be very difficult to lower them. when i had them out there is little between the base of the seat and the frame (prob easier to remove some of the foam from the top and retrim). you would have to make new side mount subframes up for them i guess. too much hassle.
welding in some fixed buckets would defo get a low position