Inflation, properly defined, is the degree by which the money supply increases. It is a useful metric unto itself irregardless of economic activity or other metrics. To claim that "GDP" and "prices" must be considered when strictly measuring inflation is to pervert the term beyond its useful definition. If you take umbrage with this statement, then please explain. I understand many contemporary economists use "inflation" to describe what is more accurately referred to as CPI figures, but many contemporary economists also think spending and consumption is productive behavior, so I'm skeptical of their prescience and take issue with their distortion of terminology.
...Which keeps things simple. Looking back, the OP has some slightly inelegant reasoning because of the "USD price inflation", which was really CPI.
Moreover, the thread has also helped me realise how hard it is to quantify growth. How do you quantify something if you don't know what it is? Why don't I know what "growth" is, in terms of the Bitcoin economy? Because I don't know what the Bitcoin economy is! I have a vague idea about some of the businesses that exist, the size of the community and so-forth, but very little concrete data. I suppose I could gather some data, give weight to various factors, and then calculate the percentage change over time, calling it growth (or shrinkage). However, anyone could argue for a different basket of metrics.
...more accurate data about the growth can't be gained without somehow separating real growth from various kinds of speculation. Even if we conservatively take CPI inflation to be 10%, and let's say two thirds of the supposed growth is speculative, that's still over 8% real, non-speculative growth for the Bitcoin economy...
There I made some leaps in reasoning that the "rate of change of mining turnover" (in USD & other currencies) = "rate of investment into Bitcoin economy" = "rate of growth
of Bitcoin economy". It would suggest that mining turnover is an investment unto itself.
Is it? I don't know. Perhaps if the miner is hoarding coins, or if they subsidise the project... However, mining turnover doesn't seem to provide useful information about investment levels in Bitcoin. And of course, "investment" is just one of many possible metrics that could be used to define growth.