Be careful with mirroring over two boxes using ASM - it can be quite a surprise for you. I have mentioned that alrady couple times on this list. See http://oracloid.blogspot.com/2006/05/vldb-with-asm.html.
Also be careful how storage chunks the space for LUNs. I know the case when Clarion was actually forming LUNs using 128K chunks and on host it was striped with 128K as well. This caused the problem in certain circumstances (and might give you headach for fine striping in ASM). I think it should be safer to use 1M chunks on storage box. At least, this is the fixed size for allocation in ASM.
Regarding number of LUNs - I would go with multiple becuase in this case you have chance to relayout easilly using rebalance feature of ASM. Since you want to chunk all things on one RAD5 group - I am sure it's a recepie for disasted... well, depending how hard you are going to kick it.
We are new to RAC and are discovering serveral uncertainties regarding the best disk layout to use for ASM. We are running two EMC Clariion systems (under RAID-5) and want ASM to write to both Clariions simultaneously (i.e. use normal redundancy). ASM will balance the load across the two failover groups (i.e. two LUNs) which reside in two EMC Clariion arrays.
The question is, whether it is better to use a single LUN with 100 GB or have a number of smaller LUNs (totalling 100 GB). Since we are using a single raid group (i.e. 5 disks) for all database storage, all LUNs will end up on the same physical devices anyway. So it might be harmful having ASM to try and balance I/O across several LUNs which all reside on the same physical devices.
This is 10.2 on HP-UX 11.23.
-- Best regards, Alex Gorbachev