No it doesen't change my opinion, in fact it re-enforces it even more. If you not after sub bass, then don't fit a sub. If you do want sub bass, then fit something that's not likely to wind up dissapointing you and costing you twice as much to get it right the second time around.
One of the best things about a ute is cabin gain. If your not happy with the midbass output from your current speakers, then by all means upgrade them to something better, or look into your current installation, things like sealing up the service holes and such other things can make a big improvement.
That sub is going to operate in a very limited bandwidth, it won't be able to play very low, and will distort very early on if it tries. A larger better quality sub will do a far better job, and vice versa some more potent mid bass drivers in the door. Again, were only talking a 1.5'' increase, you'd be lucky to get clean output below 50hz, plus you'd get a lot of fq overlap between the sub and your fronts no matter how you set your filters
What is your current speaker and amp setup. If you write back and tell me your speakers aren't amplified yet i'll shoot you.