Go with what fits your ankle/heel best. The plug and semi-plug boots have the best heel hod. Other boots have a wider ankle/heel area than the plugs and semi-plugs but can be modified to hold the heel extremely well. You say you're an advanced/expert skier so you may want to consider a semi-plug. Plenty of choices exist. Ideally, you want to try on as many race and semi-race boot sas possible. This way you know what brands fit your feet best.
I tried the boot you're looking for. I have a size 10 D width foot but fit pretty well in narrower boots. The diablo has a pretty wide forefoot. For me it was too wide. This wasn't the biggest problem,though... The liner is bulky and the boot felt like a bedroom slipper. This boot would have been ok for cruising but not for aggressive skiing. The boot was very soft, too.
Keep in mind that the boot has a lot of forward lean.
A few semi-race boots:
Atomic RT CS
Nordica Dobermann Pro 130/110
Dolomite Z Pro 130
Head RS 96
Tecnica Diablo Race 130
These boots will hold your heel better than the Diablo. I have not tried all of these boots on but i can say that the RT CS, Dobermann and Diablo race are not narrow in the forefoot.
If you like the tecnicas and ski more aggressively i would recommend the Diablo race. It will be narrower than the Mag. It has a thinner liner. Most importantly, it does not have the slipper-like fit of the Mag.