I saw somewhere they did a comparison of a separate Raid 0 and Raid 1with the IM Raid 0/Raid1 and the performance difference was pretty much negligible.
http://techreport.com/reviews/2005q1...d/index.x?pg=1
have a look through there, they basically conclude that there is no downside to using IM raid. The only problem you will have is if you try accessing data from both Raid 0 and 1 on the IM arrays at the same time, as the disk heads will have to access both at the same time, so copying data from raid 0 to 1 or vice versa etc, but generally I think the benefits would outweigh the problems. (mentioned here
http://www.techarp.com/showarticle.a...tno=206&pgno=1 )