Higorance Derek?
In a recent posting, either Mike or Ken pointed out that if a TM pump was turned down to 100psi, it would still deliver water better than a standard portable @ 100psi.
The reason for this differential is that the 100psi pumps fitted to standard portables have a much lower flow-rate than the TM equivilent.
The consequence of this is that as soon as the wand triger is opened, and water is pushed through the system, the pressure drops due to the pumps' inability to sustain sufficient flow to maintain that pressure.
Any increase of total restriction such as decrease in bore-size of hose or reduction in jet size will enable the pump to maintain pressure at the expence of the ammount of water delivered to the carpet.
The overall balance the designer considers is between this amount of water delivered and the 'Blast' effect of that water.
The above is a simplification but should give you the idea.
So increasing the bore size would tend to give a LOWER pressure at the jets, unless you fit smaller jets, which would tend to get you back to where you started
generally the effect on the pump strain would be un-noticeable.
John.