Can't Get After-market Exhaust Off? Help!

Squirt some penetrating oil in the gaps, try prising them open a bit or a bit of heat. Twist and pull

 

If it’s that hard to remove it much the worth removing the whole pipe from the bike. If you start twisting and it’s still attached at the to the exhaust port, it might be possible to bugger something important.

As mentioned a good LONG soak with penetration oil, heat and ‘shock’ movements to twist/pull it loose - far more likely to jar it free like that. Gradual pressure is your enemy.

 

It looks like there is a bracket supporting the end of the collector pipes so I doubt it would trouble the exhaust port connection by twisting and pulling. I’d try the penetrating oil first then heat the part you want to remove and maybe use some freeze spray on the collector to contract it.