I realize this would change based on how much weight it's pulling. How about using an engine pulling 3 cars? I'm guessing that sets available for sale now would have similar power. I have the engines from sets 60052 and 60098. Thanks for any input you can provide.
I've successfully run a single motor pulling two cars from 7938 and the locomotive all the cars from 79111 up a spiraling incline of two plates per track segment. I terminated the experiment after I ran out of track and table space. If the track was straight I probably could have pulled more weight.
I run my 7745, 7740, 7735 (with extra carriages), 4458 (with 2 clubcars), 4563 and 4564 up a 14 brick total height using 16 sections of straight track. On the main sections this is one brick per 16 long track section but at the top and the bottom I smooth out the incline with 2 plates and then 1 plate. (Otherwise the carriages, especially the club cars, bottom out on the track).
These are 12v and 9v models and not the Power Functions models you list so your motor and track characteristics are likely to be different. Space limitations are the reason I use such a steep incline and I do have to make sure I keep the electrical contacts clean to ensure constant power delivery or I can get some slipping and wheelspins.
Also this is likely putting a fairly heavy load on the motors, luckily I have spares and I don't run my trains for long periods of time but I don't run my 7727 on this section of track for fear of damaging it's precious red motor!
I think this is the absolute steepest you could get away with and if you have the space, two plates or one plate per track section as suggested here and in other places on the web should be your goal.