I'm sticking with Tier 3 until they fix Tier 4 (and tier 5? Anyone tested it?). T3 is very noticeable over T2 so I know it's not bugged.
I do believe the cert is only half broken. With each upgrade I did test vertical height and never really saw a boost after cert 2. Which implies the fuel is not increasing. However, you can definitely fly longer if you burst your jets. Implying that the regen is increased.
Regen testing would seem to be pretty straightforward. Exhaust your jets and record the time it takes to get back to 100% full.
As it was difficult to get good measurements from that method, I instead choose to measure how long it took to recharge 10 out of the 13 ticks. T2: 7.4-7.8s T5: 6.6-6.8s There is definitely a difference, but it doesn't match the description. If T2 gave 12% and T5 gave 48%, we would be seing closer to 5.8s for T5. At least it should give some comfort to those of us who have invested points already, knowing that it isn't totally useless.
Who uses Drifter jets anyway, I find their lack of combat ability crippling. I have to say I honestly never even noticed my 'Rank 2's' were not working better then the default setting. I have Rank 3 at the moment and now I'm wondering where I should even upgrade further at the moment.
Drifter jets are also bugged. I got the upgrade to level 2 and the burn duration is shorter than the stock drifters...
I notice a big difference with rank IV. However that could be from the regen and feathering. I want to say height as well but I'm not as certain about that part.