Been reading this thread for a few days now, just a couple of thoughts.
First, there are a few posts about feeling superior, etc. based on a certain car which reflect a materialistic bent and a poor self-image. If you need a particular car to feel complete, wow, do I feel sorry for you. Benz, Jag, BMW, Porsche, Kia, Chevy, Toyota: guess what, they are all just cars. Depending on one's focus, some are better than others, but that reflects not a bit on whether the owner is any better. After all, some of the biggest jerks around drive really fancy, expensive vehicles, right?
Second, jags are great cars - when they work.
I didn't consider one when I bought my MB chiefly because of reliability concerns and repair costs. I'm old enough to remember when it was easy to take your time looking at a jag because too many of them were simply stopped dead on the road or sitting on a flatbed at the truck stop on their way to a repair facility. Their reliability is much better nowadays, but still...those kind of memories stay with you. (And how bad were Jags? That they became much more reliable by marriage to Ford tells you all you need to know about that; of course, hearing the power steering moan and turning to see a Jag instead of a Mustang (etc.) is the true delight here.) However nice and reliable and fast and comfortable and...any other brand may be, that of course does not mean that people who are MB fans on an MB forum are going to cotton to anyone touting such brands.
Third, and primarily directed to our OP, if you think you'll see a 25% increase in HP from a pulley and an ECU, then one of three things must be true. Perhaps by ECU you mean "Extra Compression Unit", i.e., an additional engine. Or perhaps you think Jag engineering is simply the worst on the planet and there is lots of free power to be gained by simple changes (hey, pick up the JC Whitney catalog, there are lots of mods you can wire/screw/bolt/twist-tie on that will really wring the most out of your car, all while improving gas mileage, too!!). The other alternative is that you're young enough and gullible enough to believe marketing hype. Before you go down that road, get it in writing that you'll see that gain, and dyno it before and after, because that will form the evidentiary basis for a fraud lawsuit against your sellers and allow you to get your money back.
Take care and enjoy your rides of choice,