How much slower is a roaming network than a standard one? Is it worth it for me to set up a roaming network?

I am considering buying an Airport Express or two to strengthen my wifi network, but I don't know if it will do more harm than good. I read that adding extensions to networks creates additional load and can slow things down. I have 1gb fiber optic hardwired into my house, and my Airport Extreme is located on the middle floor of my 3 story house. On the middle level, I get speeds of 200mb/s through wifi which is great, but on the top level i only get 50mb/s and on the bottom level it drops to 20mb/s. I have full house in-floor radiant heating with metal fins throughout the floors, and I think that might be responsible for the speed differences.

Should I add an Airport Express to the top and bottom level? There are ethernet connections so I can hardwire them to the Airport Extreme and create a roaming network. I wonder if I will get the full 200mb/s on the top and bottom level however, or if it will slow down the entire wifi network more than it is worth. Any advice from experts out there on what kind of overall performance drop I could expect going from a standard wifi setup (with only the Airport Extreme) to a roaming network?