Can we take a short time-out and review Luffy's battle prowess to-date since his Rusukaina training:
1) one-shotted a pacifista at SA
In what was a futile struggle pre-2Y, it is a relative ease now so for sure Luffy has leveled up several notches but in typical Oda style, our power level judgment was clouded by Sentomaru revealing that it was an old outdated PX model that did not get any upgrades over the period.
At this point, I thought he should be ready to take on the New World.
2) Noah and Hody Jones at FI
Hody is just a tank, nothing more nothing less, with no significant fighting abilities. Yeah he takes a LOT of beating but that was with thanks to the steroids he took. That's all.
Luffy had to concentrate on the bigger picture i.e. stopping Noah from crashing into FI and had little to no time to take on Hody at that instance, which was an annoying pest at that. Given his disadvantages of being in water and getting a bite wound and having to use Gear Third, he did all he could and if not for the sudden revelation of the Sea Kings coming to the rescue, either Luffy would have died from excessive blood loss or FI would have gone or perhaps both.
Now, would his current new gear: Gear Fourth do any help here? Nope I don't think so. His best arsenal for that situation will still be Gear Third (needed the mass) + Koka (to negate the sea effects a bit).
At his point, I start to have a little doubt over his ability to take on New World because it took a lot (A LOT!) of effort from him to handle that situation. Don't get me wrong, I am not expecting him to be able to just breeze through the situation with a snap of finger but he ain't have a definitive "answer" to take out or do something to Noah (of course, with the annoying Hody buzzing around at the same time).
3) Caesar and his Gasu Gasu at PH
For starters, Caesar is no fighter, he is a scientist but having a DF ability with huge battle potential, both logia and its application of ability.
Luffy was totally caught offguard when he fainted from the lack of oxygen caused by Gasu Gasu. A very dangerous situation, one which could have ended his adventures if not for the plot armor.
Then thanks to the poison immunity he got from Magellan, he was able to deliver the KO on Caesar otherwise Luffy will have difficulty in handling a Shinokuni-powered Caesar. Am not sure whether Koka can negate poison.
At this point, am not sure what to say about his readiness for New World. The poison immunity is a bonus he got from his ID adventure, which greatly helped in fighting Caesar's poison. But he (and us) were not aware of how his Gasu Gasu can be used, so being caught off guard is exceptionally dangerous and he did fell for that one (can't say is his fault though but in a dog-eat-dog sea, that would have checkmated him there and then!). Still, his existing arsenal from pre-TS (plus the Koka) was enough to answer the call in PH. Just the element of surprise that Caesar had...
4) Doflamingo and an awakened Ito Ito at Dressrosa
You saw it as much as I did. Doflamingo had to be soften up by Law first before Luffy can even have a chance at him. All his pre-TS skills were useless against the first professionally skilled fighter he met. Calling on his trump card Gear Fourth, it was still not enough to put Doflamingo down effectively. Left haki-exhausted and rendered immobile, clearly Doflamingo was superior over Luffy. Only later in his rebound he was able to KO Doflamingo for good.
So with this performance, is he ready to take on New World? Not quite so I think.
Now, portraying Luffy in this way (by Oda) leaves a lot of room for improvement in which he will acquire over the course of his adventures to Raftel. It proves that there is so much unknown and unpredictable to the OP world (especially in New World) that one cannot be fully ready trained to take it all on.
His training at Rusukaina is an important one, as one must have a good command of haki in order to start thinking of going into New World. Rayleigh drills the basics of haki into Luffy and also gave some pointers on how haki could be innovatively combined with his df abilities - one which lead to the unique property of a "koka hardened but flexible" Gomu Gomu Gear Fourth.
You know what would have been truly helpful in getting Luffy really ready for New World? His hands on the DF Almanac! One which publish all DF abilities and documented applications of its abilities and its awakened status made known by past users! Knowing all the tips and tricks of each and all DF abilities would have averted a surprise like that oxygen-less knockout, unless it is a new and undocumented application of ability of the current user's innovation.
Now, I think main fights will no longer be a singular one-on-one but a team / combined effort. Gone were the days when Luffy takes on someone all on his own. It takes Law and Luffy together to bring Doflamingo down. Kaido, being a Yonko, was hinted to be even superior than Doflamingo, so highly unlikely Luffy will be taking Kaido or even Big Mom single-handedly.
So in answering the question "Is Luffy ready to take on New World?", my answer would be no and he never will, not when he thinks he can handle it on his own. He can only do / contribute so much in the New World on his own accord.