Hypothetically, MAP RIT scores can go all the way up to 300, I believe (or so the tables I've seen claim), but the spread is considered relatively poor after about 245. In the case of reading, part of this may be because of the nature of reading development. Decoding skills in the NT population naturally plateau late in middle school. After that, one should be switching to measures of reading comprehension, some of which can be administered two or more times a year (e.g., the Scholastic SRI, which can be given every 30 days, and generates a lexile score, for good or ill). Creating continuous scaling from K through post-secondary, when the basic nature of the task changes in the middle, is quite challenging.

...pronounced like the long vowel and first letter of the alphabet...