IMO the factor that has the biggest influence on a RD's performance is state of wear and contamination rather than which quality bracket it belonged to when sold.
In other words, if it's reasonably clean and hasn't seen many miles yet it's unlikely that you'll notice any huge differences from replacing it. Wear it out first, then replace it.
Besides, a 09 bike that comes with a 7-spd drivtrain is unlikely to be a financially sensible candidate for upgrades, which is another reason to go for maintenance only.
the SRAM MRX is compatible with Shimano RDs, so if you manage to wear it out before tiring of the bike pretty much any Shimano RD would do. Acera would be a reasonable replacement in a 7-spd drivetrain.