If anyone is interested in how the M Scaler works (many won't be, of course) then Keith Howard wrote an excellent
article for HiFi Critic. For those with some understanding of the fairly esoteric art of digital signal processing it's very informative indeed.
Actually, for someone knowing DSP, there is little if any information in that article that isn't absolutely well understood, and standard practice. windowed sinc interpolators are the standard way of performing reconstruction filters, and sample rate converters. The number of crossing points is a trade off of computation cost vs quality, with around 30 crossings being the normal which is basically what you get in a bog standard off the shelf converter. Which window function you use, well, again, the maths is well understood and so the tradeoffs between pass band/stop band, attenuation, ripple etc are totally understood, with a raised cosine function being the standard window.
As for convolution being O(N^2), yes, in the time domain, but it's O(N) in the frequency domain, so for long FIR filters, you normally transform to the frequency domain and apply the convolution there before converting back. I wrong a commercially available implementation of this around 2000, and it is now standard tech included in DAWs (e.g the convolution reverb included in Logic Audio -
https://support.apple.com/en-gb/guide/logicpro/lgce357aa791/mac)
I'm going to not comment on whether it makes a difference, is snake oil etc, but i'd suggest that there is nothing that hasn't been done before, and can't be done on a raspberry pi. It's totally mainstream tech.
Oh, and if you want to implement shorter FIR filters on an FPGA, there's a core for Xilinx that you just configure and it will spit out an FPGA design for polyphase interpolators, decimators, you name it. As I said, this is mainstream bog standard well understood engineering. It's is clever, but it's well understood clever, and is drag and drop (this is from 2011 for example):
https://www.xilinx.com/support/docu...tion/fir_compiler/v6_3/ds795_fir_compiler.pdf