Cutting the extensions drastically is pretty standard with the Subsonic, they're sized so that a tall rider with the pads forward still has options.
There is 40mm (~1.5in) adjustment capacity in the Subsonic bracket so you're not locked into one length. They do require more precise cutting than the Sonic bracket equipped bars. The reason for Canyon using the Subsonic is that too many people were shoving the extensions way back on the old J4 bracket and causing problems.
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