Just found this rather old thread... And I also just made about the same switch of boots. I had a old Tecnica Icon Carbon that was skied around 300 days when it broke in the plastic in front of the toe buckle. Tecnica Switzerland was kind enough to replace it with a brand new Diablo Magnesium for free. Nice customer service I must say.
Anyway, I used a 27,0 Icon before (312mm shell) and ordered a 27,0 Diablo (313mm). The Diablo was WAY to big and I had to switch it to a 26,5 (304mm) to be able to get a tight fit. No problem there though. Switching size is simple.
However, when I skied I was totally surprised. Ofcourse I felt the softness when I tried them on at first, but couldn´t even quess what a difference it would make on the mountain. The difference compared to my old Icon Carbons was HUGE! I just couldn´t handle my skis anymore. I normally ski on Salomon AK Rocket Lab 195cm for everyday soft snow use and a old Völkl Explosiv 3 190cm for harder days. The AK Rocket is very springy in feel and easy to turn and doesn´t really need much driver power to get around. But I still had problems with the very very soft forward flex of the Diablo Magnesium. As soon as the snow was tracked out and/or bumpy I lost control. It just felt like I was thrown over the bars when hitting bumps in the pist. With the Explosive the feeling was even worse. The boot just don´t deliver the kind of power that I need to get the Exlosives going, especially not when they are tuned sharp.
To solve the problem I first changed the original UltraFit to a higher Salomon Customfit Pro liner. After that I added an extra power strap above the original one to make the boot feel higher. This helped a bit and I was able to ski the boot as long as the temperatures where low (around -20 celcius). In warmer temps the problem was back and there where no other option than to buy a new boot (I asked Tecnica first for a good price on a RacePro 130 ofcourse, but they didn´t have any at stock at the time).
I got a pair of used Explosion 8:s for free from a store and used that for a while, until I found a new pair of Icon Carbons on sale. Got them really cheap and put in a foam liner from Strolz and added an extra power strap on top of it all. After testing them just a few runs I skied with a grin in my face. Home again! What a difference from the soft and low Diablo.
The Diablo however is a dream to use with small shaped skis. My soft 176 Salomon Equipe 10 3V felt ultra fast edge to edge and very precise when skied with the Diablos. With smaller skis I didn´t at all feel the same problems with the softer forward flex. But as I maybe ski 5 days a year on that kind of skis compared to around 100 days on bigger skis, I actually don´t care at all about how a boot perform with short and narrow skis.
If I had the money or connections I really would like to try the Race Pro boot. It feels a lot stiffer and higher than the Diablo Magnesium, but still it isn´t a over-stiff race version. Could be nice.