My personal rule of thumb with any organizational/productivity system (and pretty much everything I do) is "take what you like and leave the rest behind". When I look at the base INTENT of Pomodoro, it's to force me to take a physical and mental break from work so I stay mentally sharp. Far too often, unless interrupted, I can work 8 hours straight without eating, drinking, and be physically and mentally exhausted by the end of the day.
It doesn't matter WHAT I do, as far as I'm concerned, as long as I take regular breaks to refresh.
Regarding Pomodoro. I'm used to working 10 hour workdays, and I'm usually planned out two to three days in advance. I divide all my work into tasks that can be completed in a 25-minute block with a 5-minute break. After 2 hours, (4 of of the 25/5 blocks), I break for 30 minutes. So far, pretty Pomodoroish. However, with a ten hour day, I usually end up with 16-20 of those 25/5 blocks.
After two sets of four, my LONG break is 1:00 instead of 0:30, then back to the normal cycle.
The key for me is that I know my work habits. I know when I work best, what my threshold for attention is as the day progresses, etc., and I've modelled MY version of Pomodoro on MY owrk habits.
I look at it like this: if Pomodoro is "proven" to work any studies or proof would be based on average numbers over a large cross-section in order to have statistical reliability. None of us are the "average", so I feel we've got every right to make adjustments to suit our personal needs based on the intent of the system.
So if you think you have the discipline and stamina to do throw the sever-Pomodoro limit out the window and make it 8, go for it. It's not like there are Pomodoro police wandering the world of productivity management waiting to pounce on unsuspecting dissidents. :)