Depends on what you're "upgrading". IDI (Indirect Injection) engines aren't very accommodating for increasing power: there are some exceptions; I really only know of one: Mercedes OM606 [and that family]. Don't know about tractors and turbos (but I _DO_ know a little about such with road vehicles) but there's more than just adding a turbo- was the bottom end designed/spec'd to handle the increased torque, can the exhaust (and intake) really handle the increased flows and are you really able to properly control boost and EGTs?
Many manufacturers today have given model groups that offer a range of power. In this case IF attachments and frames and drivetrains are basically the same then all will work together then bumping power is possible (likely one is looking to have to tweak the ECU [if one exists- more and more this is the case]),
Less HP _can_ work for many things, just takes longer. My 30hp small-frame
B7800 has done an amazing amount of work; stuff that I cannot pick up to move I push around. I've done things that no larger tractor could do. That this machine is an IDI and has no emissions equipment (one can really tell when operating it- and it being an open station really makes it clear after operating my Kioti); IF there were one just like it that could do everything that this one does but it had emissions equipment I wouldn't care.
Buy for 80+% of what you think you need. There will always be things that you need more power for but it should be very infrequent and in such cases RENT the power. Having too much machine means a lot more cost for maintenance and repairs: bigger equipment can also more readily destroy things you don't want to destroy (larger accidents)!
I've got about 870 hrs on my [2016] Kioti. I run it like I run all my equipment: I make it work. No engine or drivetrain issues: dealer did have to reseal rear diff cases under warranty (I've got hundreds of hours since then and all is holding up fine). There's only been TWO instances of my tractor running a regen when I was wanting to stop working- I just found more stuff to do and did them; in the entire scheme of things this is a total non-issue over the course of all these hours. Entropy will polish off EVERYTHING, eventually: it'll most likely finish off ME before it gets my Kioti.
Regarding HSTs, it depends on how you have to work. I would not have anything else on my property (even my Polaris UTV is hydrostatic). I have no significant slopes and I have a lot of things to have to navigate. I have no ideological blinders in this regard. I seek out the right tools for the job that _I_ have. I have no allegiance to a "brand" as not all offerings from manufacturers are of the same quality AND stay the same over time.
Sometimes you just have to suck it up and move forward. I did that with my Kioti and found myself banging my head for quite a while in the beginning (leaking cab and some other issues) but over time this machine as proven itself to be exactly what I wanted and what I needed. Would I buy the same machine today? I'd have to research to determine that. Last I looked there wasn't a law that said that if you buy something that you are stuck with it for life. If it turns out it's not what you needed then sell and buy something else: if one does due diligence in defining requirements then one isn't likely going to miss the mark to the degree that one has to turn over equipment in a meaningfully shorter period of time than one was expecting. You'll take a financial hit for sure but how much of a financial hit are you taking by waiting to buy something? Time is money... (one can always get more money but one cannot get more time)