You have to test it yourself is the short answer, Especially if its for a job you have to deliver.
A 20 second exposure at 25 ISO will have less noise than a 10 second exposure at 50 ISO, so just turning up the ISO may not work to shorten exposures. You will probably get different amounts of noise with daylight than with tungsten (a sunset shot vs. interiors). Raw Developer does a very good job with image noise. With some of my tests RAW Developer had a better ability to remove hot pixels than the Leaf software (It also makes noise with the P45 more attractive with Phase files than C1 Pro).
The Aptus 75 will give you 50 ASA with the same noise levels that the Aptus 22 will give you at 25 ASA, so it has an advantage in this situation aside from just having more pixels.
Testing, testing, and more testing. And then test some more.
I think with image noise it is also difficult to predict how noise on your monitor will translate to a print.
Also, be very careful with exposing properly or braketing. Opening up the shadows in long exposure images will not work like you can with strobe lit images.
