I've heard that tsere and open-syllabic segol are pronounced the same in Yekkish (Western Ashkenazi) Hebrew. That said, everyone talks about how Ashkenazi is the only pronunciation with distinct sounds for every vowel (I know that they're talking about earlier Ashkenazi pronunciation, as well as varieties from other regions). Even if there is a distinction, I can't produce it to save my life if open-syllabic segol is pronounced [eː] (which I'm pretty sure it is). I intend on pronouncing open-syllabic komets as [oː] and choulom as [au] and it makes sense to have symmetry, but as stated I have trouble with that.
While a lack of sources is unideal, as one of the resident Jekkes, I feel confident that I can provide an answer.
The long and short of it is that it depends on the Jekke. If you listen to recordings of Benno Weis ז"ל, you'll hear very little distinction between segol and tzere (recorded here in שני זתים).
On the other hand, R' Michael Friedman, chazzan of Khal 'Adas Yeshurun in Ramot (KAYJ) differentiates between the two vowels.
On a personal note, I try to differentiate between the two, although I'm more likely to render a tzere as a segol than vice-versa.