What's wrong with my e-mail to potential PhD supervisors?
The statements


  what are the chances that a student with a thesis in TOPIC Y would have his application taken very seriously into consideration by a group like yours?


and 


  it will have a great impact in (sic) my application strategy and maybe in the selection of the advisor for the thesis


tell me you are not specifically interested in applying here. 

Are you interested in working with this prof or just shopping around? Giving general application advice to strangers isn't high on the list of the average professor's priorities.

(My direct experience is with German and Austrian universities. Programms with centralized admissions are becoming more frequent in continental Europe as well, but in general professors have more leeway here over whom to hire or take on as PhD students than in the US.)