Grab the tinfoil hats!
There is cetainly provision in the GSM standards for levels of service, but this is only used by EE to differentiate between the public and the emergency services.
Any differences you see between the phones are probably (i) subjective, (ii) due to config differences betwen the SIMs/accounts, i.e. different APNs, (iii) due to the size of test subjects, i.e. 2 handsets and 2 sims do not scientific testing make. (iv) due to some other technical reason, i.e. taking a differnent route through the EE core network.
I will agree though that subjectively, the EE network has progressively got worse over the last 5 years (both data and voice).
Finally, I think it's more likely that EE won't go to to court with you because they simply can't be bothered.