Ahhh, bless those folks at Porsche who "reserve the right to make constant improvements." As mentioned above, a couple other options over the course of the 944 production run can effect sway bar size in aluminum control arm variants, including 474 (stablizer bars) and more rare 456 (stabilizer bars and sport shocks, I think is limited to 85.5/86 and the old offset M030 equivilant), plus the 030 packages available beginning in 87 with the advent of late offset wheels. If your 89 NA was delivered with a 474 option package would it not be possible to have upgraded front sway and the 18 mm rear without coil overs or upgraded torsion bars?

 

Optional sway bars for the 87-89 944 NA were 25.5 mm hollow front and 18 mm solid rear. That might be the "26 mm" front bar you are looking at, not the 26.8 mm x 4.5 mm M030 front bar. The factory manual states that for these model years "part of production with M 404 supplied as standard equipment = tubular stabilizer front 25.4 mm x 4 mm dia. and solid stabilizer rear 18 mm dia." If you have an 87-89 NA with no suspension option codes and a "26 mm" sway bar that looks stock, these are probably your sway bars. They would be paired with the stock 125 lb/in front springs and 23.5 mm rear torsion bars.