It a matter of a few factors:

1. Insertion loss:  How much will the insertion loss attributed to this switch attenuate your transmitted signal?  Remember insertion loss is very frequency dependent, so it may also distort your EYE diagram.

2. Reflections:  If the switch and the PCB layout does not properly account for matching impedances for your signal traces, then you may introduce many unwanted reflections.  not only can this reduce your EYE but it can dramatically impact your Inter-symbol interference, and subsequently your Bit Error Rate.

Final answer: To ensure the switch has a minimal impact to your EYE diagram, make sure it looks like a transmission line by all respects, matched to the impedance of your signal trace.