That's correct. a boom extended to near it's max. is going to be weaker and less stiff.
This will put higher loads on the front end and rear as well due to greater leverage being involved.
Extending a boom to about 75% of it's full range is OK.... any longer and you may be asking for trouble.
As far as a 238 cm clew length sail on a 240 cm boom, there's no room for adjustment (the clew length will increase as you add more downhaul) so that would not be a good match for more than just the max. extension vs strength and stiffness issues.
Hope this helps,