The reality is, we can't answer this without knowing the sensitivity/spl rating on your speakers. The NR1608 is an awesome receiver with a great feature set but extremely modest power at 50w. This might work if your speakers are pretty efficient or it might fail miserably. If the SPL rating is 90dB or above, you should be fine. 88dB might work but I doubt it will work for anything below 88dB.

I had a 7.1 system powered by a Marantz MM8003 power amp which delivered 110w continuous with all channels driven into speakers that were rated at 86d SPL (in practice, reviewers said in room SPL were closer to 84dB) and in 5 or 7 channel it barely delivered enough power in a 20'x20'x7' room with ideal speaker placements. And my room was bit smaller than yours.

In the absence of the sensitivity measure on your speakers, I would buy the most powerful Denon receiver you can afford. Denon generally runs a few hundred $ less than Marantz for basically the same specs. Denon is made by the same company and makes phenomenal receivers with the same software and interface. I have owned Denon and Marantz equipment and been very pleased with both. The most noteworthy difference is that Denon tends to be neutral in its sound profile while Marantz is a hair warm.

The Denon AVR X4200 is available for $749, the 4100 is available for $649 refurbished from accessories4less. The primary difference seems to be in what DSPs they support. Most importantly, they both have Audyssey's Platinum room correction which is super important. Those would be your safest options.

The sensitivity on those is 92dB. The NR1608 should drive them well as those are particularly easy to drive to higher volumes.

The one other factor I focus on is usually room correction. With your system in ceiling, Audyssey and most home theater sites I am seeing are saying that in ceiling placement is far from optimal and room correction software will have a minimal impact on improving sound. You will most likely hear a lack of separation across your front channels vs what you would get in a traditional setup. There is an Audioholics article where the founder of Audyssey basically says that Audyssey won't help in ceiling setups. Stepping up to a higher level of Audyssey will likely have no impact.

That being said, this is an area I would research if I were you. Maybe DIRAC or Anthem's ARC does better with in ceiling speakers. Worth investigating and you may want to check some more home theater centric forums. You aren't the first person to be facing this.