LePanch, do you currently have a lot of flats due to thorns? With each step toward increased flat resistance, the bike will become leaden-feeling and harder to propel. I would see if the tire liners alone do the job before investing in thicker tubes and tires.

(I ask this because my brother gave me an old MTB that required a lot of effort and down-shifting to ride, and it turned out to have tire-liners AND heavy-duty tubes under the knobby tires! Totally unnecessary in my area. )