Unfortunately relics really can't be farmed. Truly random, it appears. Though, I've had great luck with various relics from the Pirate Treasure room. --- You can take my Karma, but you'll never take......MY FREEDOM!
I found a shield relic with 29% in both capacity and recharge delay. Best single stat one I saw had 30% capacity. Not sure what all can come on SMG relics but I would think 29% anything woul be worth 1% damage or fire rate --- Every time I try to go where I really wanna be it's already where I am, 'cuz I'm already there