Definitely invest in one if you can, it will be an incredible investment for you if you have 15% or less humidity out there.
Under ideal conditions, it shouldn't take more than 2 months for a plant to reach 18".
Considering how hot things are for you already, things could be looking significantly worse.
Assuming you give the plant everything else it needs, your worst case scenario with constant 90-95 degree temps is airy buds and an increased likelyhood for hermaphroditism. If you can't avoid those temps, all you can do is help your plants deal with them as best as you can. Be sure to trim off any and all branches (branches, not fan leaves) that aren't receiving light as they are the first to hermie on you. Be vigilant looking for bananas come flower. I hate to be the bearer of bad news, but with those temps and bagseed it is quite likely you will be dealing with hermaphroditism in some way, shape, or form.
Ideally, temps excluded, the plant will be healthy and stress free overall and as a result won't be as likely to hermie on you. If you're unable to control temps, make sure you're 110% vigilant when looking for signs of sacks or bananas, especially on the bottom branches/branches that don't receive light.
The temps will for sure have a negative affect on your yield, but it is entirely possible to produce some quality flowers even with those temps. Your yields will suffer, yes, but the quality won't suffer nearly as bad as yields will as far as temps are concerned.
And, again, all that being said, your plants are looking really good my man. The roots and leaves look incredibly healthy and as long as you maintain that then everything else will follow suit!