Very interesting - I assume by fat rods you mean Ralf's Power Rods?
I have the full set and find I can execute even fast rolls with the PowerRods, my mainstay the HotRods and also the ThinRods - I do find more problems with the jazz rods and the faster rolls - but given the reason for using the jazz rod fast rolls are not normally required.
I guess why I find little difference between the three main types of Ralf's rods is probably due to my own hand hold. I find the the amount of surface area contacting the skin - and the shape of that surface, and not just the mass involved also greatly affects the tone from the drum.
Playing the RWE and the Brendan White I find my own preference is for the medium HotRod - in fact Ive yet to find a skin it *doesnt* sound absolutely great on!!
I tend to find that top-end tippers have great articulation but loose much of the punch - that said I have 3-4 which do produce a great sound on bigger drums - Ralfs Blackwood and Snakewood ConcertSticks - these are nice, and also recommended Christian's ME2 and ME4. (I absolutely love Christian's ME13 for Kerry style).
Paul is right - the stick/hand/drum/skin combination makes tippers one of the biggest issues - until I came across Ralf and Christian I went through a LOT of tippers - most rubbish - most will be consigned to auction sites at some point! (hey if I can get a pint of bitter/guiness for each of them its better than burning them!!).
Its the bane of the Bodhran player - the never ending search for the perfect
Tipper. Its like looking for gold at the end of the rainbow. :)