Its something I have been testing for years, drawn from maybe 100 tunes.
The main conclusion I drew (My own conclusion personal to me and not everyones experience) Is that the Top hat makes all the difference.
The fit of the rear guide does not seem to effect accuracy. It can effect velocity and noise, depending on the material but running the numbers, it does not appear to effect consistency shot to shot. However, I found the Top hat to be much more critical.
The Top hat serves to centralise the flailing spring end at the distal end of the piston. If made correctly, centralising it about the latch rod, or centralising it inside the piston. This does impact shot to shot consistency, and depending on its weight, can play a critical part in balancing the cycle. It also can remove torque given up to the distal end of the piston.
The Guide Rod, in almost every gun I have ever tuned, is usually well centred enough. This is because the latch rod has to pass through it....which sort of enforces a no choice but to have it pretty well central at manufacture, or the gun will not cock. This being reasonable, limits improvement. From here, we can then argue all day long as regard its material. Steel giving a slightly faster spring release versus slightly more damped sound from one of Delrin. I have never detected better shot to shot consistency regardless of which i used, except for better feel and nicer sound. Also any spring resonance occurring here will start milli seconds after the pellet has gone.
The real key is get on and learn your gun and find the best pellet for it....