No need for IMS upgrade on 1998 base Boxster?

[Perfectlap]

it's not really an "upgrade". That's the wrong way of looking at in my opinion.

It's more a refresh that happens to come with some key upgrades. Fact is a sealed bearing, even a dual row, will not last forever.

Given enough time under the wrong conditions, and a dual row bearing, like on 97-99 cars, will fail as well.

The inherent flawe in this bearing design is not addressed in any IMS bearing be it from the factory or the aftermarket, they all simply attempt address longevity, some very well others not all.
But at the end of the day oil contamination and oil starvation of the bearing still appear to be the root cause of these engine failures. Today we have some options available to us that insure that oil starvation is not a problem anymore (Direct Oil Feed lines) and other options remove the bearing altogether (the LNE Solution). But you're not getting either of these perks by leaving the factory bearing in there simply on account of it being a dual row bearing.... a bearing now pushing the 15-20 year mark.

so unless you know how the previous owner took care of the car it's a bit of mystery for any car if your IMS bearing is near its end or good to go.
If there's any doubt about this, the best policy is simply to put in a new one. After all a serviceable bearing should be replaced at some interval to avoid collateral damage.