Cap numbers typically differ from money spent. IIRC, Seattle was one of the highest spending teams last year (3rd?) despite being well under their cap- that's what having a good cap number can do for you because it can raise your cap figure well above the baseline. It's a big reason why finding ways to save money can pay major dividends in future offseasons.
As far as the 89% requirement, it only requires that you spend at least 89% of the baseline amounts. So if say the baseline salary cap is $125 million but Seattle has $12 million extra coming from rollover, they floor limit only factors the $125 million number, not the $137 million figure. Or to put it another way, the 89% number is the same for every team, and is pretty easy to reach for most NFL teams.
As far as this rule creating a "spending rush", it probably won't, simply because most teams are already over the 89% requirement, and those that aren't might have to spend, at most, maybe $20 million to get there. Contracts can be manipulated to cover that threshold pretty easily. For example, if you are lagging behind in spending, you can fix that up with one big signing bonus (which hits the cap gradually but impacts the spending number immediately).