I'd disagree.
It depends on the enemy.
Not really.
It depends on the infrastructure built to support the fighting force.
There's value in standardization within the service. Why would a TL 15 fighting force maintain a TL 6 arm? To what end?
Would the Imperium even garrison a planet? Against whom? The Imperium can care less about an indigenous TL 4 tribe making trouble for the locals. That's not the Imperium's job, that's a local problem. The local can staff up, fight them, bargain with them, rent a merc unit, or leave. The local nobility might support them, but, again, it's on their dime, not the Imperium's.
I can see a TL 6 Mercenary force, but I'd debate whether a TL 6 fighting force would be any cheaper than a TL 9 or TL 12 force. They all have to eat, and arguably you can do more with a smaller higher TL force than a larger lower TL force, making day to day costs lower. Plus a TL 6 force wouldn't have any intrinsic lift capacity -- they're TL 6! "What's a space ship??"
No doubt there's a point of diminishing returns in terms of how much of a multiplier a higher TL force brings to the table, especially against a lower TL force. TL12 is probably more than adequate compared to a TL 15, for example. Why pay for a TL 15 force.
But the Imperium would pay for a TL 15 force, as it's facing a high TL opponent. Do you think that there are TL 9 Imperial Marines out there? TL 12? I think the "TL" of a Marine Unit not needing the latest Combat Armor/Battle Drees/Fusion weapons would be a fuzzy number. They could be TL 15 and still use Gauss Rifles, but they might be using meson communicators and individual grav belts. Why would they use it? Because that's the gear they have and that's how they're trained.
Because the Imperium is staffed and equipped to supply and emplace a TL 15 force. If TL 15 Marines need fusion powered grav tanks and what not, the Imperium will have the "tail" in place to support and supply such a unit, even on a primitive world. They may not bring the nuclear dampers along, but, hey, a towed piece of meson artillery would certainly make quick work of a village of grass huts, and, why not, the forces are trained to work with these things anyway. (Of course they're more likely used to working with ortillery in that case instead...) They're certainly not going to leave behind their geo-spatial mapping drones/satellites that they're map boxes interface with. "Meson artillery -- just point and click. Put the + over the grass huts, and..."