You might want to talk to the builder and see what they think. The short answer is that changing the fork rake will change how the bike rides (specifically it will change its stability and responsiveness to steering inputs) and the builder designed it with a certain feel in mind and around a certain fork size. Only you will be able to tell which you prefer though.

Bike geometry is a fairly complex topic since there are several dependent variables. Stability is mostly a function of front end geometry and the most important factors there are head tube angle and fork rake. With a 43mm fork and 72.5 degree head your bike has maybe 61mm of trail which is maybe a little long but still within 'normal'; the bike may tend to steer lightly at low speed and be harder to adjust at higher speed. You could feel this as 'cornering on rails' where it is difficult to adjust your line in a high speed turn. A 50mm fork would give you trail closer to 54mm and close to neutral where the handling will be more consistent across different speeds and should enable you to adjust more in fast turns.

Some riders prefer the solid feel of more trail at higher speed, some prefer the predictable feel of neutral trail. If you are happy with how your bike rides then probably you should keep what you have, if it feels a little vague at low speed and a little too locked in at high speed then maybe try a 50mm fork.

Here is a good link on the effects of front end geometry: http://www.spectrum-cycles.com/geometry.php