Will Sonus Faber Serafino G2 be too much speaker for my space?

Those are awesome. If you can place the speakers optimally in a room then none are too big.  YMMV…comes down to personal preference as is often the case. 

@ddlux,

''I thought, as you go up the model line, the speakers would demand more space to handle the additional output.''

This misunderstanding comes up often. The speakers don't have a mind of their own! You have control of the volume.

''However to properly set and manage your low end sweet spot, speaker size comes into play and there are room/speaker size limitations.''  

You can not manage your bass response by changing the size of the woofers. A change in size will merely alter the magnitude and frequency of the peaks and nulls, something all rooms suffer from. Fact. 

Personally, I would keep the existing speakers and with the money saved buy a pair of sealed subs that have, and this is paramount, variable phase which will allow you to place the subs where they are unobtrusive. This is how you manage the low frequencies. There is nothing else you can do to tame the peaks and fill in the nulls. Nothing. Huge bass traps will help to an extent but as you are not willing to install such, which I do undersatand, subs are needed and they do not neesd to be huge. Cheeck out the size and price of the great value SVS SB1000 PRO.

Small neat and extremely capable. I have a room a little bigger than your's and get tight, clean and informative bass that never draws attention to itself and can really rock out when desired. I have one of the SVS subs I mentioned and a similarly sized REL. The REL is a dated design that only has 0-180 degree phase flip. Avoid this type.

In addition to the link @ditusa provided there is much out there if interested. Something that you may have glossed over in my rant  is where I mentioned the nulls being filled in. It's a null right? It's cancellation, there is important low frequency info missing and no amount of speaker change or component swapping can fill those nulls, sad but true. Peaks cause even bigger problems and lead to complaints of slow bass, boomy bass and one-note bass. No such thing. The improvement extends beyond better bass, with the peaks flattened the detail that was being obscured is now evident. Improvement across the entire spectrum.

This is best dialed in with measurement, . Learn how to do it or hire someone.

Don't shoot the messanger.

@gournard most subs cannot complete with an Amati above 100Hz. You would also need a pair for the left/right channels, which puts you in the price of an Amati.

@bartsw, first off, I don't recall the OP mentioning 'Amati' and secondly, if you read my post you will see my recommendation to 'buy a pair of sealed subs'

 I'm not sure whether you fully grasp the situation or not. Subs are not designed to compete above 100Hz. They're subs! Good subs will however extend cleanly an octave or two higher to provide better phase tracking at the crossover frequency.

Please answer this question: Regardless of size of speakers, number of and size of woofers and the positioning of them, how do you deal with the inevitable room modes?

Room modes are an unfortunate phenomenon caused by constructive and destructive combining of low frequencies that all rooms suffer from. This is an undisputed and well documented fact and verifiable by measurement. 

I say again, all rooms will have problematic modes. When there is a bass source present in a room, peaks and nulls will be in evidence. With more than one bass source, the peaks and nulls will be all over the place.

So the takeaway is everybody needs at least 2 subs or be prepared for suboptimal sound. 

@gournard the Nova, Serafino and Amati was the debate. I own speakers larger than the Amati and I own subs. You need both. Here's an analogy; a pair of 1" bluetooth speaker and 24" subs does not work. Speakers with a larger cabinet and cone gives you a more full and involving sound above the range of subwoofers. A grand piano coming from a large cabinet speaker will feel more real and involving than a 1" speaker or even a Nova and Serafino. Given the setup with the integrated amp and room design requirements, that's the argument for the Amati.

Theater bassheads, like me, get room modes because they think the solution to a problem is to throw bigger subs (too big for the room) at it when you can do better with more smaller subs. An over-concentration of bass from a big sub, like a hot spot on a pan, can be resolved more wisely with more smaller subs.