I have a ko propo helios with rf901 + kr-408s receiver.
I wounder if i switch to a futaba 4pks or a sanwa mx11 transmiter, is a wise move?
How much faster transmiters are the futaba & sanwa compared to mine?
I am used to my ko propo problem free operation...
I have heard that futaba had in the past some problems with their receivers.
Did they solved their issues?

I've been using my 4pks for almost 2 years and I had zero issues.

What is you opinion about buggy transmiters do you need the transmitter to be really fast in order to be fast and consistent on the track ?
ko propo helios 2.4 ghz 901 module with kr408 receiver is consided fast enough for a buggy or shall I upgrade to a newer transmitter in order to improve my consistency and lap times ? Or tranmitter response has nothing to do with driving ?
If yes which transmitter do you recommend among futaba 4pks, sanwa m12 and ko propo eurus 904 module ?

Honestly, when you're talking about brands like KO & Futaba, you really don't need to worry about response speed, because either is fast enough you'll never feel the difference(unless you're a Worlds top ten-level driver, then maybe you would). For us mortals, any of the major brand stuff(KO, Futaba, Airtronics) will be MORE than fast enough for our needs. It really just boils down to what you're most comfortable with....

In my opinion, you can't go wrong with any of the top line transmitters from Futaba, KO, Sanwa or Spektrum. They are all great and for you and me, the fact that one is better than the other will depend only on the grip or the feel you have with the transmitter in your hands. If you have a KO and have no problems with it, I don't see why you should change to another radio. They are all more than capable of doing their job as a high competition transmitter. The Futaba problem you mention was with overheating receivers- a few of my friends had such problems with 3PKS transmitters (one resulting in a pretty ugly accident, with a large scale 1:5 driving into a marshall at full speed), none with the newer 4PKS however. I don't have any experience with Futaba for now, I use a Spektrum DX3R and I am happy with it.