Sonos offer a home trial period in most (all?) regions. Buy it and try it. If you’re not happy, return it for a refund. 
(I’m presuming you mean swap out the Beam for the Arc: you can’t add the Arc to the existing room.) 

Thanks Nik. Yes that’s exactly what I meant, I should have said. Think I will go ahead & buy it, just been talking to someone who has similar TV room to me & he said that’s the arc is superb. Thanks for taking the time to reply.

No worries about a room being too small for the Arc as long as it fits and you have the recommended foot of space at each end.

Your now spare Beam actually makes a really nice, nearly invisible streaming only speaker. Tuck it up on a kitchen cabinet, you can usually run a power cable up a gap in the cabinetry. I have one on a bathroom cabinet and it sounds great and is much less visually intrusive that the previous Play 1s I had in there.

Thanks Stanley for your great response. Love the idea of the spare beam, not sure the wife will though