Changing the jet along with changing the oil pump pressure will increase the power of the boiler. This will use more oil, in the region of 0.15 US Gallons per hour. You can only increase this as per manufacturer's instructions. Most boilers only have 3 nozzle sizes/power outputs, i.e. low, mid and high, such as in a 70/90 boiler, you can have 70,000 Btu, 80,000 Btu or 90,000 Btu. This will not increase the temperature of the water travelling through your floor as it is mixed down from approx 70C to between 25 and 35C at the manifold. You would only increase the output of the boiler if the total demand is not being met by the boiler.
Before you do that, calculate the heat losses of the room, compare that to the output of the underfloor heating circuit for that room. If could be that the losses are outwaying the heat from the circuit. Calulate the loss and deliberately assume that the installer did not insulate around the external perimeter of the room (below the concrete). This can be a conserable loss.
Calculate the following, assuming Living Room Design Room Temp = 21C and outside Design Temp = -3C:
Temp Difference = 24.
Ventilation Heat Loss = 2 air changes per hour
Length of room (m) x width (m) x height (m) x 2 (air changes) = m3/hour x 0.33 x 24 = Heat Loss for ventilation in Watts. (Add this to end total)
Fabric Heat Loss:
Floor Length x Width = Area x 0.79 x 24 = Heat Loss in Watts (Add this to end total)
External Wall Length x Height - (Glazing & External doors) x 0.68 x 24 = Watts (Add to total)
Glazing Width x Height = Area x 2.8 x 24 = Watts (Add to total)
External Doors Width x Height = Area x 2.8 x 24 = Watts (Add to total)
Ceiling/Roof Length x Width = Area x 1.41 x 24 = Watts (Add to total)
Add all totals.
Add 10% if walls are exposed such as northfacing.
Add 10% if unusually high ceilings.
Add 15% if room is only intermittently heated.
Add these to your previous total.
Divide this total by 0.709 for an emission factor and this will give you a the total demand required from the heating circuit. Compare this to the output of the circuit. If the amount is much higher, you will need to either lower the demand, i.e. insulate external walls, improve u-value of window/door, ceiling etc. or increase the output of the circuit, but not so easy with underfloor.
If above too complicated, PM me with dimensions, wall type, etc. and I will calculate for you.