There isn't much air movement within the dojo, so it can get stiflingly hot in summer - which is why people try to get a spot near the windows during that season... and then they back away from the windows in winter
Although last summer was very hot, you might be lucky this year...
If you are going for the sole purpose of grading (and can extend your stay in Japan), you might like to try the Aikido Academy (also at Hombu Dojo) -- see <Aikido Academy> at http://www.aikikai.or.jp/Eng/index.htm
-- the course allows for closer interaction between the Shihan and yourself. The normal classes are packed and less personal - i.e. it's going to take much longer if you are hoping to be recommended for dan gradings. Maybe someone who has attended the courses at the Academy can comment.
In any case, it's a great experience...
Hope this helps.