I grew up working in my father's two-story (main floor + basement) frame shop / gallery. There are definite pros and cons, but I'll address these comments:
Let's say you're upstairs working and because your chopsaw is running, you don't hear the customer come in.
Eventually, they will leave as you are ignoring them... because you don't even know they are in your store.
Get an automatic doorbell/strobe. I got one in our shop. If someone walks in the front door (even if the door is left open) the motion is detected, and it triggers a strobe light / doorbell combo in the back. You can hear it, and if you are on a big saw with hearing protection, you can see it.
Plus, somone can just as easily walk in, steal something, and walk out without you even knowing they were there.
In my shop, which has a front-half gallery / back-half frame shop, we have never had any theft. Also, in my father's shop, I am unaware of anyone stealing from him, and with the basement production it was easy for someone to walk in and not be noticed (before strobe doorbells). But both my shop, and my father's shop, were in nice shopping districts, and we carry 4-figure to 5-figure priced artwork. Also, nowadays you can get a Nest Cam (formerly Dropcam) which streams a video feed to your phone. You can not only record, but you can see in real-time if someone is at your counter. I've visited neighboring businesses, and returned to my shop when I saw someone walk in on the camera.
Are you going to be sizing your glass upstairs? Rememberhow heavy cases of glass get each time you have to carry it upstairs.
THEN, the odds are good that you WILL at some point break the glass as you are trying to carry it downstairs...not to mention possibly marring the mat or art. What about when you need to work on an oversized piece?
All of this is based on your statement that the stairs are steep. Not a good thing for carrying a customers suddenly valuable art.
My father's shop had super-steep stairs. Pretty much like a 10' a-frame ladder's angle. It was skinny, steep and unpleasant to go up and down. Having said that, we did all chopping, mounting, mat cutting downstairs, then brought those items up to the first floor after they were cut. The glass was on the first floor, at a fitting-station. Since half the weight is in the glass, its best to keep that on the first floor especially since glass does not take up much room. Also, fitting on the first floor probably would help a lot. And if you're dealing with an open loft, the size of your frame won't be restricted by the stairwell, since you can carefully lower it from a loft railing.
I hope this insight on a multi-level frame shop (albeit from 30 years ago when I started) is helpful. It is totally do-able, and would certainly free up space. You would probably want a lighter saw than a Pistorius though.
