The past two years I have used a 12' 6" 6/7 weight with 450-475 skagit heads. This year I am EXCITED to try a 450 grain Skagit iFlight which has an intermediate line for 2/3 of the head. For tips I use 12-15 feet of T-8 or T-11. I tried 18 feet of T-8 and that was a shit show. I agree with John that throwing heavier tips (T-14 & T-17) will make the fishing much less fun. Those tips are also not really likely to get you in terms of added depth until the end of the swing in the slack water when they will drop like a stone and hang you up.
What I would suggest; however, is adding split shot to your line. In fact, this year I am going to experiment with 1/16 oz and 1/8 oz worm weights in the middle of my leader held in place with a blood knot or (gasp) a swivel. 2 feet of 20 lb test - weight - knot - 2 feet of 8-12 lb test. The split shot have a tendency to pop off.
It is also true that the fishing very much depends on the conditions. I have had shad take the fly mid swing on a quarter down cast (FUN). I have also had to throw quarter upstream and mend and mend and mend to get the fly in the zone (also fun). Fortunately,with shad there is no ambiguity as to when you are in the zone!
This year I am also going to try an 11' 6 weight with a 425 grain Skagit iFlight with 10 - 12 feet of T-8 or T-11. Perhaps when the water warms up.