multiple Apple airport expresses as Wi-Fi range extenders - do they slow overall speed

Attached is a shot showing the 5g without the Box checked.

The 4th Generation AirPort Extreme........if that is the model that you have.......is a simultaneous dual band wireless router. That means that it is always broadcasting both a 2.4 GHz and 5 GHz signal that uses the same name for both bands.

You would check the box in the screenshot if you wanted to assign a different name to the 5 GHz band than the 2.4 GHz band. As it stands now on your AirPort, both the 2.4 GHz and 5 GHz bands are using the same wireless network name. Some of your devices might be connecting to 5 GHz and some may not.

You cannot turn "off" either the 2.4 GHz or 5 GHz on a 4th Generation AirPort. Both bands are always being broadcast at all times, no matter whether they use the same name for both bands or a different name for the 2.4 GHz and 5 GHz bands.

The 2.4 GHz band on your AirPort is using Channel 11 and the 5 GHz band is using Channel 161. Normally, you would use the "Automatic" setting for both bands to allow the AirPort to automatically scan and select the best channel to use, but if the settings that you have are working OK, there is no need to change them.

I thought this meant it was off giving 2g only?

No, that is not the case. As we noted above, both 2.4 GHz and 5 GHz signals are being broadcast at all times on a 4th Gen AirPort. The only control that you have over this is whether you want to use a separate name for the 5 GHz band.

IF......the AirPort was broadcasting 2.4 GHz only........(which your model cannot do).......then you would not even see a broadcast channel for the 5 GHz band.

IF......you had the most recent "tall" or "tower" version of the AirPort Extreme, then that model does have an option that would allow you to turn off either the 2.4 GHz or 5 GHz signal. In the screenshot below, you can see that there is an option to turn "Off" the 2.4 GHz network on the most recent version of the AirPort Extreme.

The same option is also available for the 5 GHz band.

Does adding, wirelessly, Apple airport expresses as signal boosters "range extenders" increase traffic over the whole home network slowing it but giving better coverage?

Unfortunately, you never mentioned whether the AirPort Express devices would be using a WiFi connection to extend the network, or whether they will be using a wired Ethernet cable connection. We'll assume that you were asking about a WiFi connection.

IF......the AirPort Express connects using a WiFi connection and extends the network in that manner, the potential maximum speed of the network is always reduced by half or more. If you are using more than one Express to wirelessly extend the network, then the potential maximum speed loss is magnified even further.

So, while the AirPort Express can improve the signal strength in a given area by extending the network, the downside is that you will have a significantly slower network. That still might be OK if you don't have a high speed connection plan with your provider.

Often the only way to find out if a setup will work for your needs is to go ahead and try it out in your home. What might be acceptable for one user might be totally unacceptable to another.

I set my 5 GHz band shut off because of reduced range

This is not possible with a 4th Generation AirPort Extreme because the setting to allow this does not exist in AirPort Utility. Perhaps you meant to say that you have the most recent "tall" or "tower" shaped AirPort Extreme, which does allow the user to turn off either the 2.4 GHz or the 5 GHz signal. Users often call this model the 6th Generation AirPort Extreme although Apple does not.

Things can get really tricky if you trying to set up multiple extenders, but that might be another discussion.