I have no issues with Series 80 slides, other than the concessions Colt afforded. The integrity of the unit is fine. They're a little more resistant to the press when 'squozing', but this can be a good thing. No Walther 99's have crossed my bench, so I don't feel qualified to advise. Most systems can be improved, however, but do to our litigious society, it behooves the pistolsmith to excercise caution.