Peter, an all in weight of 3398g should make a considerable difference to the haulage capacity of the loco.
Try it on your steepest grades with a decent train behind it.
I personnally wouldn't add any more weight than what you suggest.
I think it is safe to assume that your new loco weight is not excessive, after all, LGB's forney weighs 3400g in an 0-4-4 configuration with the same Buhler motor, so a 2-6-2 set up should be fine.
Excellent Austrian narrow gauge in North Wales.