My engine build came out ~ 19:1 & I've found this to be a good compromise. It certainly relies more on having a well maintained glow plug system & some extended glow time to start cleanly below say 35 degrees F. Have one gp go out, & that cylinder might take 5-10 seconds to build enough heat to begin firing cleanly below 35 degrees. It's started fine w/o engine heater (when elec not available) down to -7 so I trust it to start (although it's certainly better for it to use the heater when possible).
Keep in mind torque = cylinder pressure achieved & how that cyl pressure happens as the crank angle rotates thru the combustion stroke. Compression ratio is a part of the cyl pressure equation - but not the only factor.
Higher comp benefits are greatest at idle & taper down proportionally as more and more airflow is pushed thru the engine. At idle, comp ratio is a much bigger contributing factor to what cyl pressure is achieved. At idle, a higher comp ratio's extra squish significantly enhances how much turbulence is induced into the IDI chamber, thereby improving fuel air interaction, helping combustion efficiency.
With certainty, our IDI engines were designed w/ higher comp ratios to gain combustion efficiency. Among other things, IDI is/was a way to get adequate fuel/air interaction utilizing a relatively low pressure (cheaper to make) injection pump. These engines were engineered to make around/under 200 hp. As we try to make 300+ hp, we're quite a ways past original design limits.
If you never have a headgasket failure (which does happen often enough in turbo 6.5's), then the stock higher comp ratio is fine. The greater proportion of time you're going to run at high %'s of max power, the more a bit lower comp ratio is likely to show benefits in durability/reliability.
These relationships show are well established w/ performance diesel engine builders. In the continual improvement of max power, when they begin to run up against headgasket/combustion seal failures, they lower comp ratio, which lowers PEAK cyl pressure/temp for a given power level. Remember the failures occur around the peak spike of pressure/temp in the chamber. The higher the comp ratio, the higher this peak/spike is.
Everything else being equal, lowering comp ratio lessens how sharp the pressure/temp spike is. You've still got to make the same overall average cyl pressure to make the same amount of power with a lower comp ratio. (actually prolly slightly more cyl press, but with a lower peak/spike).
If you can avoid HG/comb seal failures, higher comp makes more power. So they use better gaskets, o-ring comb sealing surfaces, etc. When they get to the limits of maintaining the comb seal, they lower comp ratio & use a little more boost to gain durability/reliability & still make the same amt of power.
So the folks running performance 6.5's with stock comp ratio's & never having any headgasket issues prolly make their power a bit more efficiently. Head studs, optimized head & block deck sealing surfaces, & better head gaskets are all ways to strengthen the seal & enhance durability.
Thermal efficiency is directly related to comp ratio, so higher is more efficient. But up around the comp ratio's these engines run, the differences are less significant. If the difference in thermal efficiency was huge, then my 19:1 engine wouldn't be able to routinely do 19-20 mpg (it's averaged 19 for 20,000 + miles) on summer fuel in a 6000+ lbs 4x4 aerodynamic brick.
Summarized - I lowered my engine's comp ratio to gain a little peace of mind that my engine is less likely to suffer a HG failure when working it's hardest. Any gained durabilty is in probability of avoiding a HG or any other peak cyl press/temp spike related failure.
