1. The better trigger mod than the BX is the Volquartsen target hammer kit and trigger. Most times that kit can be found for around $60 and works much better than the BX trigger.
2. Auto bolt release mod: Takes all of about 5 seconds to perform this mod: $0
3. Extended charge handle: $25
4. Extended mag release lever. $15
5. Bolt properly headspaced: $0
6. Volquartsen extractor and spring kit: $15
7. Nylon bolt buffer: $5
8. Bolt mods that help with function: radius and polish the rear of the bolt.
Many more mods, but those are the necessary ones. For the type of shooting I do. YMMV.. A huge performance upgrade would be changing out the factory barrel to a Kidd, or similar type barrel. Stock upgrade is also a consideration. The Magpul Hunter X22 is a great stock, if one wants to upgrade to a better stock.