This Thread really has it own momentum, i don't have to answer anymore i have my trusted fellow maemo members answering for me , and i thank you guys very much, saves me some time.

But to answer the questions myself:

No i sadly doesn't have the time to make battery's and modded covers for you out there, if i had i would have no problem making a small production

I don't know if one scud is enough i haven't run with a single battery since i started making dual battery's

If you really want POWER do a dual mod, it will without a doubt go for 24hours heavy usage, my previous runtime posts (some posts back) it went for 30+ hours of moderate usage (note 1)

I would opt for a mugen cover if i where you, it is no problem modding the normal cover to fit a dual battery, but the uniform mugen cover will be nicer to hold, to have in the pocket & as i stated in Q&A and many others have stated, it is much nicer to hold and write on the N900 with the mugen cover

On runtime with one scud, users have reported getting better through the day, the 15% really counts, and under high load there is a good change that it will give you more then 15%, look into discharge graphs, short explanation is that the more capacity means that it will be drained softer then a battery with less capacity, this is something i can confirm with my 1000 diy flashlights and of course my EV's + other battery operated "crap"

Note 1. everything on: wifi search, 3G, skype + msn high screen brightness etc....