Level 30 Pupitar - my turn to ask the "would you evolve" question.
Would you evolve a level 30 67% 6/15/9 Pupitar? The plan would be to hope for Bite, and power him up to 3121 or so as an attacker. The candy cost would then be 126 (100 + 26), with a small dust cost of 26K. In comparison, to bring a level 20 Larvitar hatchling to the same CP would cost 195 (125 + 70), and 80K stardust. Hmm.
Answers
Here's something to keep in mind when making this decision: what will tyranitar's CP be at lv39? If it's greater than 3530 (100% Iv dragonite) then that's certainly useful. It's also worth noting that the gym rework may make CP obsolete as it may not be the determining factor for placement.
I personally would hold wait to hatch a better larvitar as they aren't rare 10km egg hatches, as I value ivs > candy power up cost -just me though!
Imagine the weeping and wailing if CP becomes obsolete, & that's just from me. Imagine that multiplied a few million times over. But you're right, lower IVs do mean lower max CP, & I'd wait for a better Larvitar too. Having said that, the world won't end if a lower CP Pupitar is evolved, particularly if it gets greats moves.
I would with no regrets. If he gets the offensive moveset I want then I get a great offensive mon with little investment where IVs don't matter. If he gets a defensive moveset he has a high enough CP to be placed in many gyms with zero investment. The only way for me to get absolutely nothing for my investment would be to get a crap moveset on a higher IV Larvitar evo which I would do nothing with.