I haven't personally tested it, but I would expect it to be okay but a bit soft. The thing is with that large image circle one have to pay with lower resolution. The 70mm Alpagon is the Rodenstock Digaron-W 70mm and is thus a modern digital lens of the highest performance, so there should be a difference in micro contrast.
However if you shoot at f/16 with your P45+ I would expect perfectly usable results from the Grandagon, and perhaps after sharpening and a real print there won't be any relevant difference left. The P45+ has quite large pixels which of course also plays a role in masking the differences in performance.