You said the previous owner raised the needles, richening the mixture. Warmer weather would make the bike run even richer. Needle position makes the biggest difference. I think I would drop the needles one clip (raise the clip) to lean it back out a little.
My 950 also needed choke and time with the choke on to warm up.
Once jetted properly these bikes shouldn't need rejetting. I rode mine from sea level to 12,000 feet in temps from 20 F to 100F and never felt the need to rejet. So I am always suspicious of a well used bike having to be rejetted, unless there has been a significant change in the air filter or exhaust system.
Is there any chance the previous owner switched from a paper air filter to a foam filter? That would have required a jetting change. You should also check that the carbs are properly seated on the intake manifolds. It is very easy to have an air leak there LEANING the mixture.
KTMs--300, 518, 990