Re: GO, powering up Pokemon is generally not worthwhile anyway. It's more important to gain levels yourself, since that caps the internal level of your pokemon. The CP gauge shows how close to that cap they are - as you gain levels, they'll appear to lose progress.
My brother started playing a couple of weeks ago and while Team Rocket does seem to scale, it's not nearly enough. Particularly since he hasn't had the absurd boon of a community day, which (around here) gives you so many of a good pokemon that you'll find a couple with three stars and nearly maxed CP.
What about the Venusaurs that I caught in the first (I think?) community day? Not really competitive now. For one thing IVs couldn't be rated (without tools/math), but I couldn't catch any higher than my level back then. That's why personal levels are the highest priority.
And hey, filling the pokedex is good for XP, and actually fun! The biggest strats besides that are doing legendary raids (joining those pokemaniacs shiny-hunting Regigas or whatever, you don't have to have a strong team) or doing a lot of the 12-candy evolutions like weedle/caterpie/pidgey. I think grinding evolutions is my main experience source other than raiding, and you can wait until you have a lucky egg to get double credit for them.
Around 30-35 level progression starts getting crazy slow though. I was 36 for months, and 37 isn't really much different. Powering up my best pokemon takes so much stardust that I rarely do it, even when I have the candy. To power up the weakest pokemon I actually consider competitive, a 3177 dragonite, takes 5000 stardust. That's "only" 50 catches I guess, not counting weather boost, but bleh.
Doesn't help that trading any actually-special pokemon costs thousands of stardust too, and I want to share with my friends. They often miss community days because, you know, they're in the middle of the day? And apparently there's this thing called "work"? And I guess it just sucks to be them, as far as GO is concerned, because there's no way I'd have a chance if I'd missed Beldum day.
oh and I guess you're double SOL if you can't play in an urban area :/ I still wish there were rewards for, you know... hiking. Compared to doing circles in an air-conditioned mall full of free items, I mean.
Edit: Holy crap I just checked the stardust cost to let my Metagross learn a second charge move (psychic hopefully): 75000, AKA 750 captures. looool. I mean, that's turned out super useful on my Tyrannitar (Crunch/Stone Edge), but not *that* useful...