Is there any evidence that installing this kit would increase the overall value of the PA28?
You never increase the value of a plane by spending money on avionics (or anything else actually) to a greater extent than the money spent.
This is partly because used avionics are almost worthless (because, once used, there are likely paperwork issues) but also because smarter buyers know that a plane is likely to contain a few worms, and that a lot of maintenance/repair work is shoddy, so they prefer to knock down the seller by the rectification cost of all the unknown/dodgy bits, and get them done themselves at an establishment of their choice and under their supervision.