I think the days of boring out engines to gain more power is starting to pass us by some; in the older days "no replacement for displacement" was a very true adage, and it still applies for the most part to this day.
But now we have better superchargers, smarter computers, variable vane turbos, variable valve timing, GDI, ect ect. You can take a itty bitty engine like the 2.0L M133 in the new CLA45 AMG and pump out a whopping 355hp (that's 177.5 horsepower per liter) and its achieved through the modern trifecta of Forced Induction (Turbos work best it seems), Gasoline Direct Injection, and Variable Valve Timing. That thing is putting more power than most of the S/C 5VZ-FE's on the forum do with 1.4L less displacement! Now granted, we're talking a car engine and not something designed for the more taxing applications such as the truck use the 5VZ-FE was designed for, but the point is; these days we can do more with less.
Aftermarket injectors, fuel control systems, sensors, and parts have come a long way. The 5VZ-FE is already a stout, bullet-proof engine for the most part because it was designed to be put in a truck; part of the reason many people don't rebuild their engines when S/C them is because the connecting rods are much thicker than the rods in my 2UZ-FE (4.7L V8, also designed for a truck application) and the new 3UR-FE (5.7L V8) has even thinner rods; which is why those engines don't always do best with forced induction (also something about US made rods being weaker than Japanese made ones in the 4.7L but I haven't 100% confirmed that...) Point being, the engine was built tough and with today's more advanced/smarter aftermarket parts you can squeeze more power out of the engine without even having to tear it down.
In regards to the heat comment on the thinner cylinder walls, this is actually a serious factor to consider. You need to realize that coolant temperature is not the real issue here, if the system is functioning properly then it will stay where it needs to be. The real issue is the combustion (explosion) side of the cylinder wall and the coolant side of the cylinder wall will have two different temperatures. Not to mention the metal in between will be constantly undergoing temperature fluctuations, in fact you can liken it to the engine being constantly hardened and annealed to a degree. If you understand metallurgy a bit, you don't need to melt metal to change it; steel's crystalline structure actually change at various temperatures before it melts. If the cylinder wall is thinner then the temperature of the metal will be skewed higher as the coolant has to pull the 1500*F-ish degrees being soaked into the metal from the combustion process. If the metal gets too hot and then the coolant quenches it, in the long run it can cause micro-fracturing, metal fatigue, ect. A good way to understand what I am talking about is to look at how a bad weld can fail vs. a good weld (at least this is how I learned a lot about steel.) The engine was designed to be in this sort of comfortable zone of being heated up and cooled down at the same time without too much problems, hence why materials like Titanium sleeves are used to better deal with those variances. The other thing you need to remember is that there is an actual explosion occuring in your engine, fuel and oxygen are being mixture to create heat; part of the cooling system's job is to prevent your engine from becoming a block of molten metal (a little extreme as the engine will more than likely fail mechanically before it gets that hot... but give me some poetic license here) and of course you have issues such as detonation, pinging, emission (if you're even bothering to make it compliant in the first case, though to a degree you should consider it as it can actually affect performance somewhat.)
The other thing is if you bore out the engine, you need to do some cylinderhead work as well since now you have a smaller combustion chamber than before and you're trying to get more air into the engine than may or may not be possible. With NA engines there's a mathematical formula called "Volumetric Efficiency" that's used as a diagnostic tool to find how well an engine is breathing. Now with a forced induction engine its harder to calculate this and it gets thrown out the window, but the principle remains the same, if you've bored out your engine from 3.4L to 3.6L are you sure the engine can "breathe" properly? Are you going to get the proper ratio of mixture volume in your larger engine through the stock valves and intakes or are they now a new bottle neck? This is all stuff that goes into modifying/designing/building a high performance engine. For many, its just easier to bolt some aftermarket parts for a good power bump than to squeeze every last bit of power out of the engine.
Sorry if this is sort of disjointed/jumbled up, I just figured I'd throw my 2cents in on the matter and I am trying to keep the post short.
