My 90 has a mitsu with blowoff valve, and it dumps oily air (sometimes oilier than others!) on the elbo above the turbo. The bypass valve on the 87 is a lot cleaner and quieter. I like the bigger Garrett on the 87 too, although the 90 mitsu turbo spools up quicker. I have a feeling the mitsu is overheating the air at around 14psi though...the Garrett wouldn't be nearly as bad.

Point is, if you have a bypass valve and it works, leave it alone. The only advantage I see on the bov is it appears to be cheaper, a lot fewer components than the bypass valve and its associated plumbing. If you go to a mitsu, you would have a bov. If you retrofit a newer engine with a garrett without an integral bov it might be easier to fit an aftermarket bov as opposed to rigging up a bypass system. That intake hose on the 87 is almost $200 US these days, the bypass hose molded into the intake hose probably adds significantly to the cost.