Actually, that was mentioned in google AdWords learning center. Separating high performing keywords into another ad group, use searched keywords in the ad text to improve CTR.

This is something we've been debating internally for a while. According to the Google reps I've spoken with, KW CTR does not affect other KWs in an adgroup except in content campaigns.

This would mean that having low performing KWs in an adgroup would not adversely affect the quality score of other KWs in the same adgroup.

Of course I've spoken with several knowledgeable PPC people who swear otherwise.

I personally like to split up well performing and poor performing words just because it makes it a little easier to manage my groups.

The problem is twofold. When you 'remove/replace' a well performing KW, it will have to be re-qualified. This means that your well-preforming kw will struggle and you just have to hope for the best to recover to inial levels.
On the other hand you could remove/replace the less-performing keywords. When you would replace them, also they have to be requalified. Since these are poorly qualified kw's you can expect that the min. bid on those will shoot through the roof, eventually resulting in inactive keywords/deletion.
Other keywords in the adgroup and their ctr's DO have an influence on the rest of the adgroup. Even the history of the account WILL have some effect! In order to have an effect, kw's should be active AND showing, so only when a kw/ad has some impressions it will have an effect on your advertising.

My advise would be to 1) Move the well-performing keyword to another adgroup if you want to keep all less-performing keywords. Do this on a time that your account doesn't suffer the temporary loss in volume, or 2) Let the well-performing kw where it is and delete poor-performing ones alltogether, or 3) Don't change anything in the adgroup, just raise its budget.

I myself prefer a combination of 2 and 3.

Hope this contributes to the discussion.