Octane boosters are ***** - put it this way, you can't polish a turd
All you'll do is waste money and foul up your plugs much quicker!
Either use better fuel, or lower your boost.
If your fuel lid says 98 octane and your car is a genuine US car, then that's strange. I'd say your car is a gray market import, from Europe or something? If so, 93 octane would be fine. It's close enough to the 98 RON rating.
But as we said earlier, just get your car on a dyno to verify your A/F's across various rpm/load points to make sure you're in the clear with your 93 Octane fuel.
If you can't hear it pinging on hard accleleration, then you're OK, but then again, sometimes pinging is inaudible, so the best precaution to take is get that car strapped to a dyno my friend
EDIT: OR invest in a good & accurate Wideband O2 Controller & Gauge
'89 911 Turbo Cab
Protomotive MAP ECU, Twin Plugged Heads, GT2-EVO CAMs, 3.3L fully finned P&C's, ARP fasteners, C2T head gaskets, Titanium Retainers, Turbo spec valves, springs & guides, 964 splash valves, GT35R BB turbo, GSF Stainless Headers, Magnaflow Exhaust, Full bay Intercooler, TiAL 46mm w/gate, TiAL 50mm BOV, Apexi AVC-R EBC, SPEC Stage3+ Clutch kit, Crane CDI Ignition