I built my Heavyweight set (DG X100 shafts, 2" extensions, weight matched @ 135grams, 2" extra tip trimming before trimming to length) to a 40" 5 iron length, and used 3/8" between clubs (as opposed to the typical 1/2"), so my 9 iron plays @ 2 1/2" over standard (38 1/2"). From the 9i, I only have 1/4" to my PW, and 1/4" between it and my GW and then 1/4" to my SW and LW. I think the final length on my SW and LW is 37 1/2".
My lightweight set (85 gram graphite Hybrid shafts) are set up the same way, without the extra tip trimming. Not sure of the Swingweights, but to be honest, I never really cared. As long as the overall balance is comfortable, they are good to go. Of course, 80+ gram jumbo grips and 8 extra wraps of tape do have a tendency to counterbalance the extra length.
I did try a PW at 39", just too long. Chunked everything.