My opinion is very biased and based on my own short comings....
There is no way in heck I would go from a 335i to a 128i. Given you're going to a convertible there is a weight penalty and the fact that you've known the joy of having that power on demand, I'd definitely be afraid I'd be kicking my own A$$ down the road. But I'll admit, I'm a bit of a speed freak and knowing that I could have had more would drive me nuts.
4K difference when spending 45-50K...that's less than 10%...or a nice set of wheels and tires and you get better brakes to boot...not a chance for me. Besides that you're leasing? What's the difference in payment, dinner at Wendy's? Nope...no way.