The original reason for the nose to tail orientation was that the 800kw Head End Power could easily be operated from either unit, and units could even be swapped if one failed and could not limp home. The HEP was overdesigned to the point that it would even operate with a soft ground, as was quite common in the HEP trainline when Amtrak maintenance did not quite catch up to maintain coaches. These engines are pretty strong, and the P40's were rated at 4000 gross HP, minus, of course, the HP used to operate the HEP, since the main engine drove the traction alternator and the HEP alternator in the same housing. The original P40's were road numbers 800-842. The later P42's, with R/Nos 1-116? (from my memory) were rated at 4250 gross HP. Their GE FDL engines had electronic fuel injection, and performed even better than the originals, which had a lot of teething problems.
I was in the San Diego area on a demo, and an Amtrak coast line train with six coaches (from memory) arrived with an Amtrak "pepsi-can" 12 cylinder B32-8. There was a fuel truck there, and I had a conversation with the driver. He told me that his job was to refill the diesel from that train. He also told me that "since Amtrak began running these new locomotives, his revenue had been cut almost in half." (The former power used on the train was an Amtrak F40.)
These Amtrak Genesis are also very fast. I heard confidentially from RR sources that Via Rail tested one, perhaps light and with the speed limiting software disabled, and it touched 126 mph. I also know that an Amtrak Dual-Mode B32 (with AC traction motors) was tested and reached 128 mph at the DOT Pueblo, CO facility.
I don't know what the final verdict from Amtrak will be in regard to these locomotives, but any motive power unit that remains on a roster in quantity for over 27 years, Amtrak capital funding notwithstanding, must be pretty good, in my opinion.