The ET and the GMP are rather different sticks. I did not gel well with the Gravity MP. I found it didn't have enough over the 18X20 Tour to stay with it.
However, the Extreme Tour changes things. It has more control than the Gravity MP, definitely more pop - especially on serve. But I do have 5g of lead in the hoop.
But, then again, I had 5g of lead in the hoop of the MP too.
Spin is not hugely different, but the feel is much better to me. I felt the MP was a bit of a trampoline. It did have a huge sweet spot, tho.
But playing doubles, I found it easier to create spin and power on serve with the ET, and it was better at putting away volleys than the MP was for me as it is more manoeuvrable with more pop.
I think a lot of the under powered conversation is comparing it to other Extremes.
The Extreme Tour is basically a radical Lite in Extreme clothing with Extreme grommets. It has control more akin to a Radical than an Extreme, but more pop and spin than the Radicals.
Better feedback over all with the ET. The MP is a bit of a noodle by comparison, but I wouldn't call the ET very firm - it feels comfortable to my old arm.
With 5g of lead and an over grip, my ET is still sub 330g, but it is rock solid. Very impressive as a doubles stick.
The ET also makes a great sound.