Look up the website
Henningshootsguns.com
For a wide range of add-ons for the Tanfoglio pistols. The site owner has much experience shooting competition with the Witness guns.
All the Witness 10mm magazines I have came covered inside and out with preservative oil. The interior coating is downright sticky, can't expect rounds to feed properly until it's cleaned out. I also spray the interior with dry Moly disulfide, keeps things moving freely.
Hopefully by the time you start shooting this gun the shortage of ammunition from Underwood will have eased up, they have the line of loads that best show off the full potential of the cartridge. You can achieve just about any level of power with handloads, with the Hunter 6" barrel ballistics can be awesome.
I would be inclined to find out how your gun handles cast bullets by trying some rather than relying on the 'folk wisdom' about polygon rifling. Whitworth and Martini-Henry rifles used cast bullets and polygon rifling, and were noted for freedom from fouling and outstanding accuracy. Just because Glock doesn't recommend it doesn't make it so.