I directly asked customer services if they were going to be able to cope with the huge amount of new customers due to their recent promotions. I was told that T-Mob had recently been upgrading their network with more aerials etc nationwide and that they would be using the "3" network as well to piggyback on and as such coping with the huge customer base shouldn't be an issue.
Please feel free to correct this information anyone from T-Mob, 3, etc but I'm just passing on what I was told.