A Model 3 faster than top Model S will trigger The Osborne Effect.
True - if they pre-announce it. But even then how big the effect is depends on pricing and other factors. And the Osborne effect is not the only sales effect Tesla cares about.
The Osborne effect is when people don't buy your current product because you told them an improved version is coming; it won't apply once the T3 is available so we only have to worry about pre-announcements. Tesla didn't give details about T3's performance variants; probably partly because they are still working on it, but partly indeed to avoid people putting off a Model S P90D purchase now. I agree with you that some such effect would happen if they pre-announce, so that encourages them to say as little as possible until the last moment. (However, there are other factors that push them to talk about it early, so who knows what they will really do).
But once the T3 PxxD is available, some people will prefer the S PxxxD even if the T3 is slightly faster (which is entirely possible and done by other companies). Some prefer the styling of the S, the extra space, the cachet of a more expensive and less common vehicle, or the greater cargo versatility - and it's entirely possible there will be other S advantages by then. The T3 isn't an improved S, it's a different product. Yes there is overlap, but they will sell side-by-side.
While there clearly will be some cannibalization, it's not
necessarily bad (though it could be). Right now the S is the only game in town for long-distance, EV performance. The T3 will be a second option so there has to be some cannibalization. But it's possible the margins (which is the important part!) might be similar; and even if not, having a wider variety of options will grow the market and give Tesla more sales overall.
Most importantly, a very-high-performance 3 is likely to take more sales from ICE manufacturers than it will from Tesla's S line. I think Tesla should make the T3 performance version as quick and compelling as they can.