Yep, that 300hp twin-turbo 135i definitely has some appeal....
As to when do you "tell" the "other" - man, it gets more difficult all the time. I have tried the "trickle out" of information method; I have done the "already bought it" so no discussion (that worked really well on previous wife but no so good on current/"keeper" wife); I have done the "just wanting to share the experience with you" trick with mixed success. I think the best for me at this stage, given my rather poor track record of keeping cars, is to keep quietly chipping away over a period of time. Eventually she gets to the "I'm sick of hearing about the damn thing, just go do it, as you are going to anyway". Then it's usually a "go" at that stage.
My current ploy, along with gentle hints every so often, is to think out loud about the mods I may do to the GT/CS and what they cost. The last retort to that was, "that's stupid, why don't you just think about getting what you want but it has to be a total of only 2 cars here, not 3". To which I say, OK that makes sense so I'll think about it. Don't want to rush too much or I undo the gains I've made so far.
Unfortunately, as we both get closer to "retiremant", she becomes more cautious, and I become crazier as there are so many more vehicles to own before I croak.