I just did the exact same upgrade to my 30' Crusader 5th wheel. Slipper springs and shocks. My intent was to gain 2" at the wheels, I gained 2.25" by going with much heavier rated springs than the trailer needed. When I purchased the trailer, I noted that the tires had rubbed the underside on all 4 tires. Don't know what happened in its previous life, but did not want that to happen to me. In addition, the 2" at the wheels, will put the trailer almost dead level when hooked up for towing with my RAM 2500 CTD.
Finished the install yesterday, haven't taken it on the road yet...
Marc in sunny California
2013 Crusader 260 RLD
2012 RAM 2500 Laramie CTD and 4:10