How to mention possible mutual interests when applying for a job?
Imagine for a moment that Dr A doesn't want any help and would see you as competition. Would leaving off your knowledge of and interest in T help prevent Dr A from suggesting they hire someone else? I doubt it. 

Now imagine that you include mention of topic T but don't mention Dr A. Perhaps they will think "this applicant doesn't even realize we have a world expert on T here already!"

Suggesting you can help Dr A may be a bit presumptive. But saying that you love topic T and are already working on it, and that the presence of Dr A in the department is one of the reasons you want to come to this group? That sounds very positive to me.

Is there a reason you aren't contacting these Dr As directly and saying "I'm thinking of applying to your department because I'm so excited about topic T; do you have any advice for me?" That seems like a good use of your time if the number of Dr As is reasonably small.