Yes, they are waterproof. They have a breathable/waterproof inner lining and a gusset. I've ridden with them on through fords and rivers many times and had them completely submerged. They will inevitably get wet inside if you tuck leathers or whatever inside as that will allow water to soak down into the boot. Wearing a Savannah soot with goretex inserts over the boot tops I have NEVER got wet feet. No goretex/sympatex/wotevertex lined boots of any desription are 100% infallible because of manufacturing requirements (stitching etc).
Also, goretex type stuff is not a miracle material: it will not stop your feet sweating. If your feet are hot and you stand with your boots in water, your feet will very quickly be wet with sweat (not water leaking into your boot) - the water vapaur cannot pass out of the boot and evaporate because outside the boot is already 100% humidity. Many people confuse this sweat effect inside goretex clothing as a leak in the clothing (especially round the crotch of goretex-lined trousers). It's not a leak, it's your own water wapour condensing on the inside of the garment.
Triple T:
Buy with confidence. They are definitely 3-4 hours in the rain dry (caveats above noted). They are also, as you say, great looking boots, very very comfortable for me at least, and extremely hard wearing. I also find that the stiffness/clumpiness helps with good positive gear changes on the GS. Now when I ride occasionally with race boots I feel like there's nothing on my feet and I have to be much firmer changing gear.
Gazza:
My Sidis are going to need re-soling. Where did you get yours done?