Okay since noone has answed you i will try to give it a shot.
Let start with your suggestions
How I think to solve the problem:-
1. Create a Voice SSID and connect all laptops and smartphones on Floor A to that.
2. Change the existing 10/100 switches to 1 GB switches which have QoS options in them.
3. We'll be getting a FotiGate firewall - so configure VoIP QoS in that as well.
4. Change our existing ADSL Broadband connection to a Leased Line.
Number 2 is valid that could be part of the problem.
Number 3 and 4 i got a question
Do you have the voice gateway on the internet? is not internally in your company?
If it internally in your company how does the calls works? do they work fine? or you still got issues?
Number 1 yes you could try to create a voice vlan but im not aware of what it does... not it does says specifically how does it work...
Doubts:-
1. Does doing the 1st step only would solve the issue - nothing else is required?
2. Does having QoS on IAP, and then on Switch, and then on Firewall adversely imapct the performance?
3. Is only changing to Gbps swtch would be enough - no QoS is required?
On number 1 im not sure i havent used this voice vlan never so i wouldnt know.
on number 2 It should not impact the performance if you configure it correctly.
on number 3 not really
Now i got some questions for you
Are you sure you dont have any source of interference? around?
Are you sure the issue is on the BW side? and not interference?
Are you using Band Steering? are you using just application for voice? or you using also wireless phones?
Did you notice that if a client is connected to 5ghz band does he has the same issue?
Anyways
defenately you have to change those switches as you will never reach max AP BW capacity with those.... it suposed you got 300mbs speed for each band wich is a total of 600 mbs and in the port you got 100mbs limit right now...
Do you have a free uplink port to test? when i mean uplink sometimes on some swithces they are 24 ports 10/100 and 2 ports which are 1000 and if you using just one for the uplink you might be able to plug one of those AP to that port and run your test on that specific AP.
Sorry im not that much hep but i have not used IAP almost never... we always for now have managed to sell aruba solution with controller, which it has more options...