As the title suggests, furnace minecarts remain capped at vanilla booster rail speed even if they are travelling over powered maglev or lightspeed rails. They will also forcibly slow down the entire train if other cart types are coupled to them. One would expect them to work in one of two ways:

1: Furnace minecart can only use its own power to accelerate up to vanilla booster rail speed, but can be acted upon by maglev/lighspeed boosters to achieve faster speeds

2: Furnace minecart is able to slowly accelerate to maglev/lightspeed rail max speed on its own

Have you tried using the aerodynamic upgrade on the furnace minecart? I think that may help.
I'm going to look into this.

I just re-did some of my tests with aerodynamic upgraded carts and the issue persists. I don't know if it would make a difference but I'm using the mod as part of the "Create Above & Beyond" modpack

Alright, I'm going to look into this and issue a patch soon.

Much appreciated

Thanks

I hate to bring it up again, but the latest patch doesn't seem to have fixed the issue. To make sure that there weren't mod conflictions on my end, I did some tests on a clean profile with only the latest build of More Minecarts installed:

Github wouldn't let me import a higher quality video, but I think it's enough to show that no matter what rail the aerodynamic furnace carts are on, they are capped to the same speed.

Huh, that's odd. It certainty seemed to work fine in my test world. I'm going to look into this a bit more.