The difference between the standard and elite array is about 10 points for a +1 CR, so going from 15 to 20 would call for a CR increase of +1, if you increase the CR at all.
What level are you anticipating they fight him at? The base stats and even traits lose their impact as the levels go. Personally I just pick the stats I want for my BBEG's, and they are usually tougher than the PC's. I also don't adjust the CR. That being said I like to push my PC's and let them risk the chance of death regularly.
well the advanced template on a creature is +1 CR, giving +4 on all stats and +2 natural armor. Without other adjustments I do not think 15 to 20 point buy would impact CR at all. If you give them PC treasure and scores you can add +1 to CR, even if you use that extra money on permanency spells or the like.
What level are you anticipating they fight him at? The base stats and even traits lose their impact as the levels go. Personally I just pick the stats I want for my BBEG's, and they are usually tougher than the PC's. I also don't adjust the CR. That being said I like to push my PC's and let them risk the chance of death regularly.
+1 I like the idea the a group of characters has the chance to meet just about anyone in the realm of play. that encounter could be lowly level fodder or crushing might of the realm. What they choose to do during that encounter is entirely up to them. This mentality has all but stopped the hack and slash approach from the players. Kinda gives them pause to think "should I be swingin a sword or running for the hills" it also forces narrative from them while they try to answer that question.
That being said I really do not have much use in CR's other than to ballpark the level of the encounter. Half the time I improve the encounter anyways. The important thing is that I always have a way out for them if they decide to play it that way. In other words I never sit the group in an unwinnable situation with out at least one way out. They do not always take that way though and this can really open up some new role play channels. "now that you have surrendered to the bandit leader you attacked, you find your selves stripped of weapons and gear and have your hands tied behind you, a heavy rope noosed around your necks links you all together in a line. From the way the bandits expertly assembled you as captives it would appear that they are slavers not mere bandits." Hopefully this is near the end of a session to leave the players stewing about it for a week. (evil grin)