Lee , 90cm is easy , there is still spare space at the bottom so I would only have to shorten the exchanger from 50 to 45cm , you probably won’t lose any heat at all !
As for max temp at the brush it differs depending on your system .
I use 2 LPM through 100 microbore then through Gardiner red fan jet pods , I usually get 35 to 40 at brush in winter and 45 to 50 in the summer .
What is your setup , I can give a better idea then .
