The pedal systems are really nice across the board. Having to constantly work on higher end bikes I see a ton of pedals and pedal issues and we just don't have issues with Shimano SPD-SL pedals as much as the other brands. I switched to them from a long history of just about everything else many years ago. I started with the cheap end (you mentioned 540 - they probably were) and moved it up. Ended up with the last series of all metal Ultegras. Bought the carbon DA ones when they first came out. Bought a used set of carbon Ultegras like last year as well. I still ride those DA's to this day. They are like brand new. I had to pull and service one as I thought I was having a momentary bearing issue - I wasn't. not only was I not but the bearings were basically brand new as well. They are so well designed and the quality of the assembly is just so far superior to any other pedal system on the market.
Those pedals have already outlasted multiple bikes and will continue. I have the Ultegras on a backup bike and the metal Ultegras still being used on the track bike. The old cheap ones - my son has them but hates using them so we switched to something easier for him.
I personally think the upgrade to at least Ultegra level is completely worth it. Buying them from the UK you will spend less than we do at wholesale level so definitely worth it. It's nice to never have to even think of your pedals again. They are definitely a component that should never have any thought put into them after buying them. IMHO