I like how you shortened the muffler! But you should redo it to match the size coming off the turbo. More of a pain now then issues later on. You want the highest flowing exhaust after the turbine. Ideally zero restriction... When you bottleneck it like that it will still run but your robbing quite a bit of HP and can create heat/detonation issues on long pulls.
The turbine (hot side) operates by a pressure ratio, for a given exhaust pressure "in" you can achieve the highest pressure ratio across the turbine by having free flowing exhaust on the "out". With the less backpressure on the "out" the differential is greater and allows the turbine to do a better job at spinning the compressor (boost). When you have more pressure on the "out" it restricts the turbine wheel and makes it work harder to do the same task. Try and minimize this restriction and it will improve performance.
