Some data: http://paste.ubuntu.com/12887493/ (same data is below, but for formatting the paste is nicer).

The problem stated (and generally known) is that hwe-t kernels do not reliably boot on power8 in powernv ("bare metal" mode).

There are at least 2 possible solutions to this problem:
 a.) fix maas to be able to set a subarch ('hwe-u') for enlistment and commissioning.
 b.) remove the 'hwe-t' stream data from existance and hope that maas then uses the 'hwe-u' product in its place.

'a' has been reported as to be fixed in 1.9, and thats great, but that does not help users for the short term.

For 'b', this is still less than ideal for several reasons:
 1.) hwe-t (3.13) kernels work fine on ppc64 kvm VMs
     If i were a user of maas driving ppc64 kvm VMS (PowerKVM or Ubuntu), then I'd be quite happy with 3.13/hwe-t which has support until 2019.04 .  I'd much prefer staying on that supported kernel to the "interim kernel upgrade path" [1].  Ie, If I installed a hwe-u kernel that kernel is only supported for 10 more months (2016.08).  If we remove that product, we'll no longer have a way of installing 14.04+hwe-t.
 2.) If we remove the 'hwe-t' stream, we're essentially deleting a product on the server side and hoping that maas systems will follow that deletion.   I'd hope that maas doesn't do that, as it would mean a error on the server side could completely wipe out installable systems on the target.  The code i've written for mirroring streams does not delete products locally unless specifically told to.  So, likely an existing user *still* has to do something manually to fix this problem.
 3.) its not clear if this would work or not.  Small amount of testing would get us an answer, but not clear yet, and I personally didn't understand how it happens in code.


[1] https://wiki.ubuntu.com/Kernel/LTSEnablementStack#Kernel.2BAC8-Support.A14.04.x_Ubuntu_Kernel_Support

-- data in line --
$ fmt="%(product_name)s %(kflavor)s %(kpackage)s %(subarch)s %(subarches)s"
$ daily=http://maas.ubuntu.com/images/ephemeral-v2/daily/streams/v1/com.ubuntu.maas:daily:v2:download.json
$ releases=http://maas.ubuntu.com/images/ephemeral-v2/releases/streams/v1/com.ubuntu.maas:v2:download.json

##
## From daily, we have trusty builds for only hwe-t, hwe-u
##
$  sstream-query --max=1 --output-format="$fmt" $releases ftype=boot-kernel release=trusty arch=ppc64el | column -t
com.ubuntu.maas:v2:boot:14.04:ppc64el:hwe-t        generic  linux-generic             hwe-t  generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t
com.ubuntu.maas:v2:boot:14.04:ppc64el:hwe-u        generic  linux-generic-lts-utopic  hwe-u  generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t,hwe-u

##
## From daily, we have trusty builds for each hwe-t, hwe-u, hwe-v
##
$ sstream-query --max=1 --output-format="$fmt" $daily ftype=boot-kernel release=trusty arch=ppc64el | column -t
com.ubuntu.maas.daily:v2:boot:14.04:ppc64el:hwe-t  generic  linux-generic             hwe-t  generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t
com.ubuntu.maas.daily:v2:boot:14.04:ppc64el:hwe-u  generic  linux-generic-lts-utopic  hwe-u  generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t,hwe-u
com.ubuntu.maas.daily:v2:boot:14.04:ppc64el:hwe-v  generic  linux-generic-lts-vivid   hwe-v  generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t,hwe-u,hwe-v