It's been a few years, but I have had alot of 318's on my bench and do'nt recall anything about "bad deck design".
Granted the 50" deck is'nt the most common (in my experience) But I really do'nt think it's blade tip speed, I'm a little suprised you did'nt see any improvement with the high lift blades. The gussets you mentioned are a bit of a memory test, Are they on the outside of the deck reinforcing the front wear bar? or are you refering to the skids (maybee).
How do the baffles under the deck look, should be one continuous one in the rear shaped a little like a "W" and one in the front forming somewhat of a circle around each blade contributing to the suction of the blades. are they in good shape? Is there any large amount of grass build up, which is common if your cutting damp grass (which according to your comments is the case) If you coud get/or look at a parts breakdown for your deck you could see if all your baffles are there. A missing or damaged baffle from the "get go" may have been the problem all along.......Check the "wings" on your blades if they're worn you will lose lift. But sounds like you've been there done that with the blades. Don't forget about the the obvious stuff like belt tension, PTO as well as mower drive, PTO clutch air gap (although after this amount of time if the clutch gap was off you would have certainly burned it up) Engine power, both cylinders firing (left bank as warm as the right bank) OK that's enough of that, Yes, Gates belts work good for your car but not so good on your mower (seen enough of that) Anyway, no back brakin' here, just a few thoughts.
