Back to the 6.5
I personally think that if you are going to build a custom 6.5 that the 18:1 pistons and some serious boost are the way to go.
I'd have to agree. The AMG/Peninsular Engines 6.5L TD I had installed was significantly better than the stock 6.5L TD (Early '92 manufacture). At first, I was a bit disappointed in the ability to use all the power in the replacement engine (230HP vs. 190HP stock), but the issue was a defective turbo, which failed eventually; the replacement has given me the engine I expected. With the original turbo, I would hit the EGT wall fairly frequently, even on moderate grades. With the new turbo, I've yet to be able to hit redline. I attribute the performance to the lower CR pistons.

The 18.1:1 pistons give up nothing in performance over the stock CR, and economy is up 15%. Now, I don't engage my 17K lb. motorhome in drag racing, but it'll suck the doors off of gaso-powered RVs in the same size range.