Pete,
I believe it would be unwise to remove the C3 or C4 in the context of the TimeStep power supply; as this is a capacitor which sits across the 21V supply to the main power feed to the motor drive circuit IC and also to the stator coils themselves!
In fact I have increased the value of this cap to 100uF using a quality Alluminium foil type. The 21V also of course has to feed other elements of the circuit, but in many cases these are stepdown voltages via further regulators; which I believe the super duper Paul Heynes power supply addresses.
The Timestep power supply does have a reasonable 4700uF across the rectifier bridge output, before the LM317 regulator. However the capacitor across the output DC is a modest 100uF.
If one wish’s to improve the PS further then they should look at replacing the rather loose and ineffective co-axial connector, the possible intermittent contact resistance at this point could cause a loss of integrity of DC supply.
In fact I consider the best connector is no connector!!
Directly connect the DC into the SL1200 with a better quality twin cable.