I have not tried to change tuners as i had no reason for going with conexant as i need the best performing system.
I had many receivers but right now i also have a Pace 2600C1 Sky box which is claimed to have the best tuner of them all, it's a older Sharp tuner in that as well and i can say that my IPBOX outperforms it which surprised me.
My previous experience is that most receivers with Sharp tuners got the best sensitivity, they make great tuners, then it's up to the software makers to take advantage of it.
Just look at Enigma2 for DGS/IPBOX, the sensitivity is really bad compated to original and 3rd part images based on DGS.