Make sure you have the correct coil installed. Refer to your manual.
Generally speaking, the coils usually are not the cause of weak flippers.
Didn't hurt to replace it but probably not necessary.

Did you check your bushing to make sure is wasn't cracked?
When you say " rebuilt flippers", did you replace the plunger, link and stop?

See Vid's guide here: https://pinside.com/pinball/forum/topic/vids-guide-to-upgradingrebuilding-flippers

Keep looking, you'll find it.

I'm pretty sure there should be a capacitor installed on each flipper to reduce spark and prolong EOS switch.

If you operate the flipper mechanisms manually, does the weak flipper feel like it has more resistance than the other one?

What is your voltage at the flipper coil lugs? You should be seeing around 75 vdc if memory serves me.

Powe on
Meter set to 200 VDC range.
Take the black lead of your meter and clip it to your ground braid. Red lead to one of the outer lugs of the coil. You should see the same reading on both.
You may see some fluctuation. A volt or two up or down. Should be in the 75v range.

20150715_194802.jpg

Have you checked the bushing ro make sure it's not loose or ctacked?

Take Vid's advice and jumper the EOS.
This will either eliminate or this part of it or show that it's the culprit. Don't cradle the ball too long while it's jumpered!

I'm really out of ideas here. What sort of incline do you have on the game?
I don't own a Taxi so I'm not too sure how steep is too steep for that game. I would think that at 6.5 degrees, you should be able to make either ramp with ease (providing you hit the shot correctly).
I'm thinking your game might be a good candidate for a flipper upgrade. Have you read through Vid's guide?