Upgrading an MSA 2000 G1 to P2000 G3

[eci]

Per:

http://h20195.www2.hp.com/v2/GetPDF.aspx/4AA0-8284ENW.pdf

it says:

When using heterogeneous expansion enclosures (6G and 3G) 
the MSA2000 (3G) expansion enclosures must be installed as the 
last enclosures and straight-through cabling must be deployed.

Can you elaborate on "must"? I would like to upgrade my existing 
MSA2000 G1, 4 enclosure setup. I am installing two new P2000 G3 
FC/iSCSI controllers in my existing MSA2000 G1 SAS 3G enclosure, 
as supported. I have to do this to upgrade the array and 
preserve data. I am adding 4 new P2000 6G enclosures. The 
devices will be installed top down like this:

1: P2000 G3 array head in MSA2000 Enclosure
2: P2000 6G enclosure
3: P2000 6G enclosure
4: P2000 6G enclosure
5: P2000 6G enclosure
6: MSA2000 3G enclosure
7: MSA2000 3G enclosure
8: MSA2000 3G enclorure

I do not want to use straight through cabling, I want to use 
fault tolerant. I understand that the B end of the array ahead 
will hit the 3G device first, while the A end will hit the 6G 
device first. I am willing to live with this. My question is on 
HP's use of the word "must" in the above quote sentence from the 
guide. Is there a reason other than slower performance why I 
can't use fault tolerant cabling here? Is there a critical 
reason? I have another functioning system using fault tolerant 
cabling in the same config without issue. If it is just " the B end will run at 3G" then I am fine with that. I can make my existing 3G vdisks owned by B, and new 6G vdisks owned by A. B can provide failover for A, at reduced speeds until is back up.