Hey, I just want to say that I have installed trim-tabs on my 170 DC and love them! I have the same power setup as you DF 70, 15 pitch. Now I can plane out at 10-12 mph! It is unreal how much better my boats manners are since the tabs. Now I can adjust any listing isuue due to uneaven loads, push down the bow in rough water, less bounce etc. I could go on all day about trim-tabs on my 170! check out the inst-trim 10x8 for your 170, best investment ever!
http://www.insta-trim.com