Questions regarding global MX and NS records

[Atkins, Brian (GD/VA-NSOC)]

After specifying MX records for a 2nd tier domain, is it necessary to
restate the MX records for a new $ORIGIN? For example, if I have:

$ORIGIN .
...
        IN              MX      10 mx1.example.com.
        IN              MX      10 mx2.example.com.
        IN              MX      10 mx3.example.com.
        IN              MX      10 mx4.example.com.

Do I also need to do the following?:

$ORIGIN blah.example.com.
	MX	10	mx1.example.com.
	MX	10	mx2.example.com.
	MX	10	mx3.example.com.
	MX	10	mx4.example.com.

Or is the first statement sufficient to cover all sub-domains as long as
the MX records are the same?

Also, I have a number of records that I redirect to a GSLB device and
have 1,000+ records that I delegate to the devices as follows:

$ORIGIN blah.example.com.
www                     NS      gss1.example.com.
                        NS      gss2.example.com.

Is there a more efficient method of doing this, eliminating the need to
do this for every sub-domain? Perhaps a forward statement in the conf
file?

Brian