Does your "client" have any weld techniques for welding this material.  Most power houses or power plants have a weld technique which shall be followed.  For instance I was working at a nuclear power station and we had a specific technique to follow for every different type of material that we welded.  I know what you mean when you say "carry a-lot of metal" that was the way that I was taught to weld, the hotter and wider the bead the better, especially for x-ray work!  But certain techniques will only allow you to weave a certain width such as three times the diameter of the welding electrode including flux for a 8018B2 welding electrode and the maximum recommended bead thickness of 1/2", which is alot.  I would think that any stresses created during the welding process would would be taken care of during the stress relieving process.  But again these are all questions that should be addressed and resolved by the client.

If you are filling up a joint of a specific size, and the net heat input is the same (ie when one big pass puts the same heat & metal into a joint as two smaller ones), the residual stress and distortion will be smaller in the joint with the larger passes, so I do not think this is what you need to worry about.  The problem with P91 when "carrying a lot of metal", is that thin passes are recommended to aid in tempering of the previous pass. If a weld bead is untempered by the next pass, it will remain untempered martensite until PWHT.  This can lead to cracking if the situation is right (high stress, &/or defects).  Some websites that have information about welding P91 material are http://www.btwusa.com/html/tech_tips.html

Metrode also has a good amount of information on welding P91 whether it is on their website, or they will provide written technical documents also.