If it is really a Cooke Triplet, I could only expect that the 2.8 would be worse off than the 3.5, as they would have to make compromises to make it work at 2.8 at the expense of performance at smaller apertures.
The first top-of-the-line Kodak Instamatics (700, 704, 800, 804) had 2.8 triplets (Ektanon) in lieu of the highly-appraised Ektars (Tessars). It hurt not only the performance of those models, but did huge harm to the whole Instamatic concept, as Anti-Instamatic zealots quickly assailed the Instamatic cassette itself (despite the fact that a less-expensive Instamatic, the German-made, match-needle 500, which did have a Schneider Xenar, produced superb images).
Note that it is not in all cases that the 2.8 version of a particular camera is a compromise with relation to the 3.5 model. The Stereo Realist came in 2.8 and 3.5 versions - but the 2.8 was a vastly superior camera, worth five times as much as a 3.5 on the used market in the 1990s. The half-stop improvement was trivial, the difference was that the 2.8 had Tessars whereas the 3.5 had Cooke triplets that were not sharp when wide open and gave dark corners when closed down,