A jack-up kit will definitely have no affect on the peg position as the pegs are mounted to the main frame, I also doubt it will have any discernible affect on ground clearance given how close the pegs are to the swing-arm pivot. Having said that it will improve handling if you don't have a height adjustable shock, will allow you to stretch your legs out a bit more while sat at a standstill, and the bike will feel a better fit given your height, so is worth fitting anyhow.
Tony Archers are very reasonable for seat work, I paid £85 for a complete re-foam and re-cover - https://tonyarcher.co.uk/services/bike-seats/
Tony Archers are very reasonable for seat work, I paid £85 for a complete re-foam and re-cover - https://tonyarcher.co.uk/services/bike-seats/