I had an opaque IMS on my ’00 YZ, worked fine, right side-only petcock cleared my high and very large PC header and I have no real complaints. Some people criticize the fact that they only use one of the two mounting bolts (right side I think) but this was never a problem. I raced it, rode it, abused it for about 18 months, by which time it was no longer opaque, but yellowish brown all over. That seems to happen to all opaque tanks eventually.
Then, for no real good reason other than not wanting to put that ugly thing on my new ‘01, I shelled out for an Acerbis tank, white instead of opaque. I originally wanted the Davis model but they were on backorder, and once I saw an Acerbis in person I decided to get it instead.
It probably isn’t as slim as the Davis tank, but it must be close, it is noticeably slimmer than the std. IMS model. It makes up for this by being slightly taller, but mostly by carrying the fuel lower like the Ty Davis tank, which is why it needs two petcocks where the IMS only uses one. It also comes with a machined filler insert so in the future you have the option of running a dry-break system without having to buy a new tank (you just need the new dry-break filler).
Caveats: The right-side petcock on the Acerbis will not clear a PC T4 header. The tank will not even mount on a 426 with this header installed. It is pretty close to the stock ’01 header (say, less than an inch), so I kind of doubt it will fit comfortably, if at all, with any aftermarket header.
Having seen a few in person, I would imagine the Davis tank shares this trait. So that is something you should look into if you don’t have the stock header. If you are worried about this I’d say get the std. IMS, it should clear any aftermarket header.
Hope this helps.