Been through this too many times on the ESprit forum, but here goes on thisforum.
Little elcetric driven fan may be capable of moving a few litres of air a seond, lets say 120l / min or 2l/ sec.
The engine is 2.5l, each cylinder opens up to accept fresh air & fuel every two crank revs, so that's 1.25l/ rev, at idle plus a little bit (1000rpm) that's 1250l/ min. Anyone spot where we're going.
The fan can move 1/10th of the required air volume at just tick over, so in effect it's already restricting air flow, so it's not going to push more air throuth the engine, and result in more power, if anything you'll notice a drop.
Maybe it's a REALLY big electric fan, but normally they're not.
In short.
They're SH*T, and a complete waste of money. Better off spending the £5- £50 (depending on which one) on the injectors being cleaned, ither by a bottle of stuff from a auto factor or being removed and cleaned ultrasoically. the spray pattern will improve, as will combustion, so better engine response and more power (closer to original, not more than original).
Andy