Have to disagree Tim,..Big baitdroppers are total cr*p in fast flows, because of their imbalanced design, ..I have adapted mine with a bigger lead on the front.
It is not about the size of the dropper, it is all about the balance. The bigger baitdroppers require so much lead to stop them kiting,.. you need a spod rod to cast them out!
The Thamesley design has worked for me on the Avon and the Wye because it is a balanced piece of kit,,..as they say,..it's not always size that counts!
I've been using baitdroppers for the past 35 years ,..shop brought, and my own in rivers up and down the land and have never found anything to surpass the Thamesley design.
If you cast a shop brought big baitdropper into a big river carrying extra water it is a lottery as to where it hits the riverbed, whereas a smaller balanced design would sink quicker and hit the spot,..and would also make less disturbance and not need a broomstick to cast it.