Does Spell Resistance scale when advancing a monster?

I just advanved a Canoloth (MM 3, p. 200) as I wanted to use it against a lvl 14 party. It is originally CR 5 and I wanted to raise it to CR 8. So I added 4 HD (CR+2) and the elite array (CR+1). This raises a lot of stats, especially saves, but against a lvl 14 party Spell Resistance 18 is miserable. Effectively the Canoloth lost half its Spell Resistance compared to basic Canoloth matched against a lvl 11 party. So would you judge that raising SR to 21 would raise the advanced Canoloths CR above 8 or should this just be part of the advancement to CR 8?