I get what you are putting down with the idea of improvements at the time of rebuild, this is standard hot-rodding practice and has been going on for years... At this time I have yet to wear out a Porsche or it's engine, that would take more time then I have into the marque already, not that getting a car and driving it that much isn't a nice prospect but it takes time. So for some of us it makes more sense to throw some money down up front and buy what you want to drive now, not for 10 or 20 years from now.No. A "standard" Porsche engine was 100HP/L in 2012. 991.1 Carrera S engines were 105HP/L. 991.1 GTS engines were 113HP/L. 991.2 GT3 RS engines are 130HP/L, granted the price of what it takes to get there is brutal. Although the 9A2 Evo is neat, it's still "built to a price", which is a price at or below that of a base 911. 103.5HP/L means it's time-tested, cost-effective stuff.
My cost to upgrade to a 718 Spyder, as I've mentioned, is ~$42,000 all said and done. That's ~$1075 per additional HP. Yes, it's more than "just HP", but $42K for incremental upgrades is the definition of setting money on fire. I didn't buy my first Spyder by doing things like that, and I'm still mad at the bit I lit on the Cayman. Keep in mind that eventually a car will succeed the 718 Spyder. Do you buy that too? Wait, that one got succeeded as well -- crap, better buy that as well. Some people do this, and have the means to do this, but I'd hate to be a miserable unhappy person that never has "the best" and is 6-digits poorer to show for it.
The reason you wait for a rebuild is because a rebuild using stock components is going to be $18-20K as it is. Most of that being labor. Since you're already spending the money, you spend more on what's possible at the time since you're already in there, as well as anything that addresses any issues or weaknesses that are uncovered in the future. The extra cost won't be trivial, but you can get the cost more around $300 per additional HP because you're mostly just paying additional money for parts at this point.
This is what people do with classic car engine rebuilds. They do upgrades because they're rebuilding the engine anyways, and already eating huge labor costs. Ergo, my idea is to just wait until my engine is approaching the point where it needs a rebuild. Doing it any earlier is just setting money on fire. Buying a new car is also setting money on fire. I'm patient. New internals don't exist yet. I am not dissatisfied with my car. Ergo, I wait, and will continue to wait for a long time.
Regarding you HP comparisons, your examples are all 911 based engines, we all know that Porsche is not going to release a new engine for their entry level sports 'car' (718) and have it make more power than the current 911 model, IIRC the 3.0l Turbo in the 911 make 450HP, so the 9A2 Evo was carefully planned to come in below that number... the 911 pole position must be maintained!