When a member chooses to "opt out" of a war, will their base count toward your clan when searching for a clan to battle?
Supercell has not publicly released their matchmaking algorithm and they probably never will.
So at this point it's based on conjecture and user tests. However, from user tests, it seems that once you have your number set for the clan war, matchmaking uses just those bases that are chosen for the clan war.
There's a thread on supercell's forums addressing this, but there is no fully conclusive answer for this.
From my personal experience the clan wars that we have are not as high level. I mean we have an opted out player at level 110, he is in masters league. Other players are low level about lvl60. As we know that clan wars are based on upgrades(how much is a base up upgraded ). We generally are opposed by players level ranging from 82 to 58(statistics from last clan war). Here Max level opposition 1st player is not high as 110 level opted out player we have. So I think opted out players don't count toward selection of clan war.