Over a year since anybody commented hope people still look here. I get a bit confused by the Mbyte and mega bit thing. Using a speedtest I am told my internet speed is at maximum 4mbps but I think they mean 4mbytes per second. Knowing this am I correct in thinking getting the latest high speed homeplugs would be a waste of money as I would never utilise their speeds?
If you are using one of the popular Internet "Speed Test" sites, then it's unlikely they will be using mbytes.
The link rate (speed) of data networking technology is usually cited in bits not bytes, not least because it is a measure of time and not quantity.
"Speedtest" doesn't actually measure the "speed" (rate) of anything, it performs some measured data transfers over an arbitrarily chosen time period, then computes a statistical average, (like the trip computer in a car,) but it is also expressed in bits not bytes. So 4mbps from Speedtest is 4 megabits per second, not bytes.
It's important to understand the the journey between the Speedtest web servers and your client device "hops" across many distinct network links and each has it's own characteristics and performance (and varying traffic levels.) Speedtest measures the entire journey end-to-end complete with all the accumulated vices of each hop in the pathway. On some particular day, at some particular time.
It used to be the case that the slowest hop between source and sink would be one's local Internet link, but as "super fast" broadband rolling out, that is not always the case. Also, we cannot assume that everything "upstream" of our ISP link is of infinite capacity - there's plenty of constraints there too.
Whether faster HomePlugs are worthwhile depends on use case. If the HP's are the slowest link in a pathway (whether it be to speedtest, local traffic or anywhere else) then it may be worth trying faster ones (if your mains is up to it.) If the HP's aren't the bottleneck, then it would make no difference. It needs to be assessed holistically.
A faster set of HP's won't magically "fix" a slow Internet link in the same way that raising the speed limit of a road in Glasgow does nothing to address the speed of traffic on a congested road in London. If London is where the bottleneck is, that's where you need to address the problem if you want to get to Glasgow quicker and your journey starts in London.
Of course, "speeding up" the slowest hop in the pathway is a movable feast. As soon as one fixes slow link A then it simply shifts the honour of being "slowest link in the pathway" to another hop - which may or may not be a "problem."