Based on my own progress with building my MJOLNIR armour in this manner, I'm just now finishing up with my helmet. Granted, that's due to the fact that I didn't devote as much time as I would've liked, as occasional trips to the hospital and many late hours at work ate a good portion of my spare time.
If I'd been able to do, say, 3-4 hours of work each night (I'm talking Monday through Friday, plus all day weekends) I would definately be much further along with my build. If you really commit yourself to the task, you should be able to get yours done in a relatively short time. I don't know if it would be in time for HALOween, but probably just after.
Now, to answer your question of "is it only a little slower than say Pepakura?", well... it all depends on who you ask. I've seen some people just breeze right through thier Pepakura build, and others take significantly longer. In a case such as mine, I would say that it does
take a little longer, as first I printed out/folded/glued the Pepakura model and then began adhering/layering the styrene over that.
I guess that bit of info also helped to answer your second question, "did you use Pepakura templates or did you modify the design?". Regarding the second part of that question, as I'd mentioned in my previous post here, since I am using the HALO 2 Pepakura template (before I would've had access to the HALO 3 one) some of the details -- ie. indent at bottom-front of visor, vents at top-front of helmet, etc.) I've had to put a considerable amount of time in cutting out what little detail was there in the template and replace it with more in-game accurate details.
Rather time-consuming I know, but when done it'll be very rewarding, and I'll be a whole lot happier with the end result.
Pertaining to pics, unfortunately the digital camera I purchased a little while back, although nice (and was on sale), is USB2 whereas my old piece o' junk computer (five years old now, yeesh) is only USB1 compatible. So, I could take pics of my work/progress thus far but unfortunately would not be able to transfer them to my hard drive and then post them here *sigh*. I am hoping to rectify that situation soon enough so I can show others my work.
Anyway, I hope some of what I said above is of some use to you...