I used 14 / 50 gearing on my 350 EXC-F for a roadbook rally in Morocco. Topspeed of the bike was about 140 km/h. What is a "comfortable" cruising speed depends. I had no wind protection on the bike (no rally fairing) and was standing on the bike most of the time. I found that I can ride 70 - 80 km/h for a long time, everything faster was tiring because of wind drag. The bike could handle more than that, easily. So I guess if you sit on the bike or have decent wind protection, you should be able to cruise at 110 km/h or more.
If you want a bike only
for rallies, I'd go for the 450 EXC and kit it out with parts from Alberto Dottori (www.albertodottori.net
). The 450 has more torque, which helps in sand and mud. It is only about 6 kgs more than the 350. The reason I bought a 350 and not a 450 is because I want to use the bike not only for rallies, but also for technical riding.
Either way, both the 350 EXC or 450 EXC are excellent bikes, you can't go wrong with them.