Thank you all for the responses.
In general I'm fine with my Arius DP and I think this thread confirms much of my thinking. I'm a multi-instrumentalist and studied music education 20 years ago at a university. I spent a lot of time sitting in practice rooms fitted with Yamaha U1's and similar, in various states of tuning and maintenance, mostly while primarily practicing other instruments.
My Yamaha Arius C71PE's keyboard feel is in the neighborhood of those pianos. One thing I've become more conscious of in the past 2-3 months is that there's tremendous variation among real pianos of different makes, models, and eras. In terms of real performance, there's a mental advantage to not being so locked into a certain feel. I recently played a Steinway concert grand at my old college and the keys felt loose and imprecise ("broken in"), I perceived latency (it's a big instrument!), and the sound sounded very spread out (well, again, it's a big grand!). But a trained pianist told me that, "wow, this piano's great!" Whaaa! It took a few minutes to become familiar with it, then it just became another piano - albeit a big $40k+ piano. I've also had the pleasure of playing on a consumer Baldwin console piano from the 1980s. Again, a very different experience.
So my standard for a DP is whether it's good enough to learn good technique on, so that when I perform on other equipment my skills will translate without much fanfare. (at a piano recital, it's not uncommon to be playing on an unfamiliar instrument, and rarely do you have an opportunity to warm up on the piano before performing) Part of playing piano, to me, is accepting that I'll need to be adaptable to different set ups.
But we're also talking about what's a good enough set up for home use and whether there's an opportunity to improve it with just a modest investment.
I found the P515 comment regarding the sound quality interesting. I'd imagine my Arius's sound system (if it were a PC) would be akin to While my Arius lists AWM generation and CFIIIs samples. I reckon this really quantifies as mid-2000s processing power, imagine a Pentium 4, 512gb ram, and a smallish sample library 256mb. If Yamaha used even 10 year old technology in their CFX sound, imagine what you might be able to do with a Core i5, 4gb ram, and 2gb of samples.
The other factor remains the meh state of affairs for built-in speakers on DPs. My main question was really whether the internal speakers of a DP might be able to drive Pianoteq sufficiently - the general sense I've gotten is that I'd get better results with my own external speakers. Fair enough. I have some decent speakers to play with so if this is the best way, I'll make due (I have KRK Rokit6 monitor speakers, though I worry about their weight wearing down the DP, plus they're way too close to my head than ideal; I've most recently been using a small amp with some vintage Optimus Pro LX5's; in either case I probably should think about creating a shelf or get some free standing speaker stands.).
Lastly, given that it's rare to find a good showroom to playtest the different permutations of features and functionalities, we often fall victim to specs (and online forums!) as the best way to understand the landscape. I've stepped into a Guitar Center recently, and more often than not they don't have the model you want on display. So much of we do relies on looking at specs and making our best guesses. And particularly in the case of used instruments, I can't do an apples to apples comparison - usually there's 1 instrument to play on and I'd have to do my best to evaluate the DP in isolation.
The key here is doing this all on a budget. With a music educator mentality (geared toward inspiring others to get started and push to the intermediate level), I want to help advise parents (of child learners) and adult beginners on how to get going without a big investment upfront - they can spend the money saved on lessons.
For $400 or less, I'd feel reasonably pretty good recommending a used mid-2000s Clavinova (or Roland or Casio) with a GH/GH3 or equivalent would be a better buy than almost any new DP at that price range (let's say a Korg B1SP, anything Williams, Yamaha P45, Low end Casio, or what folks often start with, non-weighted Yamaha or Casios). And they'd still have to grow with MIDI and software instruments.
So regarding the CLP-340. It meets or surpasses the specs of my current DP. I guess all that's stopping me is $150 and whether the wife will be mad that I'm taking up a lot of space with another piano. But think of all of the dueling piano opportunities...