Interesting. I have never played ME3MP over WiFi vs. Ethernet on the same hardline, but my experiences with UMTS (weekdays) vs. DSL (weekends) are consistent with what you are proposing. Although the nominal transfer speed of the two connections that I use is nearly identical, the UMTS one is atrocious with lag and finding games on ME3 while DSL/Ethernet is usually fine. With normal internet usage, there is no perceivable difference.
I suspect that it has to do with packet loss, but damned if I know if there is anything that can be done about it.
It could be packet loss but EVEN WITHOUT PACKET LOSS you will get lag on MOST wifi connections. (The exceptional case being that yours is the only wifi router for about 1000m and you have a single user - you - connected at short range. This is not the case for most people.)
The worst part about wifi is that the lag is beyond your control because other routers will cause verification collisions that will cause jitter spikes, exactly the things that are hardest for the netcode to compensate for.
For most people, who live in wifi-noisy environments, the only way to minimise lag when using wifi is to have computer and router close enough (< 1 or 2m) that you could easily plug in the cable anyway.
Yes, but I'm fortunate enough to live in LA and have a good connection (with AT&T... bleach...) that top 30mb/5mb down/up with a very low ping. So, most of the issues were WiFi related for me... And I do some work at home that require a better connection intranet anyway....
That's all good, only it really isn't going to affect your in-game lag and I don't want you to be disappointed 