Is an 06 Legacy sedan or sedan gt strut suspension a direct fit with a 98 subaru legacy outback?

No! Been there, done that. The first iteration (1996-1999) OBs were soft handling predominantly because of small wheels and spongy over-tall Michelins (205/65-15). The very best way to improve handling is to stiffen the shoes and socks and leave the ankles and knees alone. First you have to ask yourself if ground clearance need stay the same (which to my mind is too tall)? If so, grab some nice cheap 17" alloys and put on the 225/55-17 setup used so successfully on the 2006-2009 OBs. You will be very impressed. If lowering ride height is preferred...and you have $$ for wheels...you can have ultimate handling with 215 or 225/50-17 and drop an inch or so. Compromises for potholes would suggest 225/60-16 (the 2000-2004 OB setup); this would still be on the soft side, however. If you're going to chase wheels I'd definitely jump to the 17" and then decide tire aspect ratio for ride height between 50 or 55. Note that the 205/50VR17 was used in sportier Legacy Wagons and Sedans in 2005-2009 to great effect. Essentially you're just dropping your first edition wallowy OB back down to where it should've been. As you blow struts you'll find the OE replacements are nice KYBs, so that's cheap. Leave the springs alone. I've improved body control significantly on 2006-2009 OBs with really nice aftermarket sway-bars made by Rallitek. I do NOT know if he makes thaen for the 1995-1999 Leg/OB chassis. It's worth a call, as fatter bars will plant your tall mass very nicely, adding much security to the now-surefooted shoes after the wheel/tire mod. Just throw those old 15" wheels away!!