What kind of carb is on your 450 ?

I don’t know anything about the website, but is the jet kit from the carb manufacturer ?  If not, don’t get it.  Get genuine jets or jet kit from the manufacturer.
There are lots of kits sold without markings of any kind on the parts, or different numbering systems, making jetting and tuning difficult or impossible.
Wasted money.

A great source for genuine carb parts is Sudco.  I have used them for my primary parts source for 20 years and trust them.  Sudco.com or call 1-800-998-3529.  They are in the West coast time zone.

Since you plan to change the air filtering system and go with straight pipes you may need to go up on jets and adjust the needle height so be prepared with a larger size of each jet and perhaps adjust the needle.
Straight pipes lend themselves to the upper rpm range with a decrease in low-mid range performance.  You can get mufflers that allow free flow and give good mellow sound without being harsh.
Take a look at DragBikeMikes talk on exhaust/mufflers; he talks about this.

Six Sigma is similar to Dynojet kits. They are usually sold in Europe.