I can understand the concern especially when you're expecting speeds to be vastly improved with a package upgrade. That said though the issue appears to be with the RT-N66U Dual-band 2x2 N900 you're using. Though it's marketed as an N900, it isn't actually capable of speeds up to 900Mbps. That marketed number is actually the total speeds combined among all 3 antennae. To obtain the theoretical maximum divide the marketed number (900) by the number of antennae (3). This leaves us with a 300Mbps theoretical max which is then reduced further (nearly in half in most cases) to determine real world speeds.
The speeds you're receiving over WiFi now would likely be greatly improved by removing the 3rd party router as the CODA4582/U modem is a 4x4MIMO Wave2 Gateway and thus capable of wireless AC speeds of up to 1Gbps.