I have an MR3 and I also have GT4000. And an old Alvey fibreglass that gets a run occasionally as a spare reel! There's no getting around it, the MR3 is expensive. I think the upgrade is mainly aesthetic, both in quality of machining and finish as well as 'feel'. The drag really is beautifully smooth. You have a fish on and it's taking short stabby runs a few metres from the boat, sudden stops in between. You can play the fish one handed if you want to, because the drag is so good. Fish takes line, fish stops, reel stops, still tension on the line. (Of course if the fish runs toward the boat then the rod holder has to get to work). Palming the reel is completely unnecessary.
I know in the past couple of years we have seen more products in a middle price range that claim similar high-end drag systems without the high price of the Islander. I can't speak to how well those work. But bear in mind they have effectively set the Islander as the benchmark for quality and performance, they all sport large diameter drag systems and CNC machined anodised aluminum bodies. It's somewhat akin to Harleys and the host of V twin cruiser bikes all trying to look, sound and ride like them. Some will treasure 'the real thing', others just see them as hype and image and cheerfully enjoy the less expensive but similar performing product. And just like Harleys, the Islanders have the odd annoying little trait (line creep) that owners put up with as 'part of the experience' and detractors consider unacceptable in an expensive reel. Lots of charter operators use Islanders because of their longevity, that gear gets run more in a season than recreational users put on in a decade. Quality is there but it's irrelevant to most sport fisherpeople. But you can go on a charter and 'test drive' the expensive gear to see if the lovely 'feel' they have is enough to make you lay down $600.
In the end it's personal preference. From $100 to $600, they all do the job. There will be as many opinions as there are salmon fishermen.