I know the turning circle of the 110 is worse but is it really that bad?
No, you get used to it and after a while you don't notice it, you just drive accordingly.
Am I going to lose significant fuel efficiency (TD5)?
No, from what I've seen the difference is negligible.
Does a 110 take forever to warm up in the morning?
No although the heat takes more time to reach the rear seats the front is just the same as a 90.
Is there any appreciable loss of stiffness in the extra length?
Not noticed any.
Is a 110 worse in any appreciable way?
No - it is better as a family car - just the turning circle is worse.