The good news is that strength 8 is not meaningfully more effective than strength 7 against land raiders. A squad of 3 kans with rokkits has an almost no chance to get an immobilized or weapon destroyed result against a land raider. As an Ork player, you just aren't going to kill land raiders at range, and rokkits aren't gonna change that. Rokkits are meaningfully better against AV13, but they're still pretty ineffective due to low volume of shots, and AV13 is pretty rare. What you need to worry about is AV10-12 spam, which lootas handle much better, and even grotzookas do a reasonable job against AV10 and 11.
Hmmm...I never thought of it that way before. I may drop a squad of kans w/ rokkits for more lootas. I don't have the 3rd set of Kans yet, so there's still time to change something if I want.