The center channel with my system is seamless and does an excellent job. Dialog is crisp, easily discernible and has not let me down in my short time spent with the Dali's. The 800's IMO are designed for larger rooms. That is one of the reasons besides cost why I opted for the 400 series. My room is only 1500 CF.
The differences according to Dali's website are:
"Basically, the H400 is a 2½ way bass reflex speaker with 2 x 6.5" bass/midrange woofers and the proprietary DALI ribbon/soft dome hybrid tweeter module."
"Essentially, the 800 is a three-way bass reflex speaker with 2 x 8" bass woofers, 1x 6.5 midrange driver and DALI´s proprietary ribbon/soft dome hybrid tweeter module."
"The C200 is basically a horizontal Helicon 400 - the same drivers and Hybrid Tweeter Module."
Also, you may want to read this review if you haven't from Perfect Vision.Dali 400 Review
The difference's between the Helicon 800's and 400's should not be an issue. It's a very good center speaker. It's not a wimp by any means, but my suggestion if you have concerns is to attempt a demo at your local Dali dealer.