Pilot Jet 35

The ULS Fuel pump is a Constant Pressure, Variable Volume pump.
The Fuel pressure is determined solely by the spring in the fuel pump.

A smaller bypass hole would just bypass less fuel at the same pressure.
The purpose of bypassing fuel is to ...
1. Keep more fuel moving In the hoses; not allowing enough time for it to heat up enough to "Vapor Lock"   and... 
2. Quickly return any vapor bubbles that may form in the fuel to flow back to the tank instead of the carburetors.
There is no advantage to reducing the return flow.

Making the bypass hole larger would return more fuel, decreasing the vapor lock possibility but also causing the fuel pump to pump more fuel at the same pressure.
The pump is theoretically capable of moving ~10cc (~1/3 oz.) of fuel per second, it would be able to keep up with a hole twice the recommended size.
With a hole large enough, the fuel pump diaphragm would eventually be moving its full available stroke instead of its normal small partial strokes.
This would likely cause the pump diaphragm to fail prematurely. 

The 35-Jet has been found to be a reasonable compromise and does the job adequately.
Leave well enough alone.

Thanks Bill,

Sooo! when taking-off and in climb out ie high power/RPM & fuel consumption, the size of the return aperture, has no bearing on fuel system pressure?

Correct!  Until the point where you exceed the capacity of the pump, the pressure (at the Pump) will remain constant.
The increased flow to the Carbs may drop the pressure slightly due to friction losses, but that is not pump related.

The pump pushes fuel via the big spring in the pump.
The pump operates on a cam on the Prop shaft.
At 2200 rpm the pump is cycling just over 15 times per second.
At 5800 rpm it cycles almost 40 times per second.

The pressure is not exactly constant.
The pressure drops to zero on the pump intake stroke.
But the pump typically is making pressure 90% of the time.
As the flow demand increases the pressure cycle decreases.
The fuel pressure actually flutters very quickly every second.
This is of no concern as long as the average flow rate is able to keep the carbs full.

Again, as long as the pump can keep up with the flow requirements of the carbs plus the bypass flow, the pressure remains relatively constant.
Academically, the Peak pressure remains constant; the average Pressure varies slightly.
But the pressure is not the object of concern;  The FLOW Volume is what matters.
As long as 5+ gallons an hour can be pumped to the carbs, the pressure used to get it there is not important.

Many high-wing aircraft can likely be operated on gravity fuel flow without any fuel pump installed.
I do not recommend trying this as high nose-up climbs may not work well.

Bill - on one point;

I understand the concepts of pressure & volume/flow, however from practical, in cockpit, monitoring we use pressure as an indication of fuel supply "health". So when pressure drops we get concerned about flow ie supply of fuel to carburettors.

I Agree!
As you stated, "...we use pressure as an Indication of fuel supply Health." 
The key word here is "Indication".
Pressure is way easier to measure than Flow, so we use pressure as the compromise indication of flow.

I tried to find a typical Performance/Pressure chart for a reciprocal (Diaphragm) fuel pump, and none are to be found.
There are plenty of examples for constant displacement (Fuel Injection) pumps, but none for constant pressure pumps because the chart would just be a straight line until the flow limit was exceeded.

The original question was, " Would using a smaller jet increase fuel pressure?"
And the answer is No.  Diaphragm pumps are constant-pressure pumps at least up to the flow limit.
And using a #35 Jet for the bypass orifice does not approach the flow limit of the Rotax fuel pump.