Sure thing - 10 years is quite closer call for total domination in Shogun for my experience, but this level (by my experience from Sengoku Jidai campaigning) is reachable by only quite economically strong and powerful clans - like Uesugi or Hojo. Shimatsu and Mori starting development would be slower - then their times is closer to 15 years. Oda, Takeda and Imagawa have very many opps from start - then in "Fair Campaigns" (no reloading is allowed) - 20 years is great achievement.
Mind you - I never use Daymyo and Heir killings, maybe it is my fault, but it seems quite out from game-athmosphere. If Leader of clan is old - according to Sengoku Jidai custom he would adopted somebody. Excuse me for reminding - Uesugi KENSHIN couldn't have ANY biologic heirs, then idea of whole giant clan crumbling, because you kill all sons of Daymyo in question - is rather laughable thing.

I haven't experience in MI/WE environment, then 5-years for total domination is as good number as 50-years for my perception, but if you've got 2 different stacks of 16 units and send them by different roads you can conquer 2 provinces in each turn... Ehm... Maybe...
Well, you will get quite problems in Sikoku Islans environment where you would need to divide your second stack for obvious reasons, while your main army would get some problem in Nagato bottleneck. Hm-m-m...
I don't know environment, but 5 years for total domination is GREAT ACHIEVMENT indeed!