5-10 hp with an intake/catback.
not worth it IMHO, unless you want it for sound.
now, a blower, header, intake, tune... you will be in for a good increase.
there is a groupbuy on teh 2.7l headers from thorley (again from customtacos) and a catback groupbuy... there are some dyno sheets and vids there as well...

89 4Runner
3" BL, M/T locks, 33"bfg mt, bilstein, Kayline, tubebumper, toyotafiberglass panels
TBI: Elocker,3.4 w/ORS,b+b,S2Sstg2cams,arias pistons,P+P intake,TRDs/c,URDpullies+7th,downey headers,MAPECU2,WEGO WB, SupraMAF,walboro255,stg4clutch, EPaOo2 sim