Vacuum to the modulator softens the shift. Typically the modulator vacuum is somewhere around 12" at idle. I'd expect more vacuum to make the shift even softer, or if the modulator is maxed out at that point, make no difference at all. Try replacing the cap and see where it gets you before going nuts trying to fix a problem that may not exist.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)