Trouble with concrete floor and roaming.

I work for a small business and naturally, wear many hats. As the one with the most experience and comfort with technology, I'm the de facto IT guy. This generally works out fine.

We have about 6000sqft of office/workshop space on the first floor, and a 1200sqft mezzanine as a 2nd floor. Mezzanine is floor is concrete on top of corrugated metal, supported by metal girders.

I have the 6000sqft covered with a Linksys EA8300 pretty well. And I can cover the mezzanine with a Linksys RE7000 but only with separate SSID's. This has worked for a while, but now that we have a new VOIP system, the boss wants to use a mobile device softphone as his main phone and he wants to be able to roam about with out switching AP's manually and still receive calls. I'm comfortable telling him he can't roam while he's on a call, but it would be nice not to have to switch AP's each time we swap floors.

The problem I'm having with roaming is not one that I have been able to find other people talking about. Concrete, metal and WiFi do not get along, we all know that. But what if when on the 2nd floor, devices see the downstairs signal as being strong (they usually show up with 3 or 4 bars), but then when you connect to it, the bandwidth and stability is crap? This part doesn't make sense to me, the floor should be killing the signal right? The 1st floor AP's signal shouldn't present that strong, but then crap out. Please correct me if my understanding is wrong.

When I've attempted using matching SSID's and settings with or with out 802.11r enabled, but devices typically choose the 1st floor's AP instead of the 2nd floor AP when upstairs. Even if a device initially picks the 2nd floor AP it usually swaps over to the 1st floor's without moving around.

I'm open to buying more appropriate equipment for the situation, but I don't know what would be the right AP's to buy. Or if they'd actually have settings to handle this. I've read about the Linksys and Ubiquiti AP's but really can't tell if they'd actually be of any use here.

Any thoughts, advice, or more info needed?