I have two inexpensive DVD players, a Denon and a Yamaha.
Yes it's true that a dedicated CD player with laser optimized for CDs might give a tiny bit better results than a DVD player's laser optimized for DVDs, but I think the player's internal DAC and analog output stage will be the biggest determining factor in sound quality.
For those that have a DAC with S/PDIF digital inputs, the DVD's sound quality will be more similar to the computer source using the player's digital output. But with a USB-only DAC like the Firefly, I think you'll find that having to use the budget player's internal DAC, the sound quality will likely be a notch below what you're getting now with the iMac > Firefly DAC.
Steve