Glad to hear the good news! You may want to eye-ball or measure the front/side/rear clearances between the water pan and the cooking chamber walls/door to make sure it is adequate...it may be just a design flaw that never got straightened out.
Then, check the flame color between the various heat ranges of lowest, medium, then high. It should be completely or nearly completely blue on low, with increasing amounts of amber color with a higher flame...if mostly amber, this is a very in-efficient flame and there is a problem with the air/fuel mixture (too rich)...not enough air getting into the gas burner venturi/inlet.
Also, look for excessive leakage of smoke from the cabinet/door...this will cause alot of heat loss as well.
Be sure to try different vent setting also...if you have lower intake vents, try closed, 1/2 open, full open positions while watching the temps...do the same with the exhaust, only never fully close it...1/3 open to full open with most verticals.
If the above items check out OK and you still have problems getting temps up with the water pan in, then I'd say go with a slightly smaller sized pan if possible...the pans act as a baffle to deflect and distribute the heat outwards for a truer indirect cooking. If the clearance is too small, the heat may not pass around the pan fast enough to carry the thermal energy upwards throughout the cooking chamber.
Good luck and good smokes!