Tiffin didn't really upgrade the power - the ISB they use just got more powerful as years progressed.
The GVWR for the chassis is fixed by the chassis builder, but the actual weight with coach body on it varies with the body style, floor plan, equipment (std or opt), etc. Thus it is individual to each specific coach and shown on a federally-required label on the interior wall by the driver seat. Snap a photo of each you see when shopping, so you can compare them later.
In all likelihood the 34 footer will be somewhat lighter as built and therefore have more cargo capacity and perform a bit more lively, but I would not conclude the longer models are poor performers or very low CCC. You are wise to check that, however, and recording that info should be a standard part of inspecting any rig that you are considering. It's a key factor in your decision.