I have the DP 4.5 w/HD192, and PT with an Apoggee Rosetta 800. The Rosetta is a much better unit than the HD192, but the HD192 is a much better unit than most converters out there in the prosumer range including the 2408's. I know first hand that many film composers use Logic 7 w/2408's. And they have complaint lists about Logic bugs that are very long. But not about the Motu interface. The HD192 is no joke unless you're a mastering engineer. I am not saying you can't hear the improvement of a mytek. When I got my appogee I was really amazed by the improvement, but my recordings with he HD192 were fine for a 16bit CD release. If you can't get a decent recording with the HD192, you might want to look at other devices in your chain or room issues screwing up your mix.(not critizising or saying you don't know what's up, just a thought here)
As far as song writing goes, DP is a very strong program, more so than PT and Logic... With all the midi plug-ins and the key remapper really that comes in handy. PT is easier to edit and mix in my opinion, but I think a DP 4.5 w/ a HD192 is an excellant choice for a song writer and producer. The grass isn't so green on the other side. The price tag is there for the greener grass too. If you have the cash for mytek, go for it. You'll be happy with the results.
My recommendition is to keep everything the same, save your money and work on your mix conditions and signal path going in. Is your mixer noisy etc. What exactly is wrong with your recordings? Really, until you can get good recordings with what you have, buying new stuff will only slow down your production. Try recording into the HD192 directly without the console in the chain. I use a speck 5 for direct box. No console needed.
I use ProTools for all my plugs, for pitch correction I use Pitch Doctor by wave mechanics (sound toys). For Audio units you might look at Antares, but there are tons of companies making pitch tuners.
Bottom line, my recordings made with the HD192 in Dp4.0 held up well playing back in DP4.5 with newly recorded material made with the appogee and running it all under the Protools DAE. I have always monitored through the benchmarc DAC1 and I can hear a marked improvement with the appogee.
Never owned mytek, but hey, they're mytek... they're obviously pretty great. If you can afford it, get an external clock too. I am adding the big ben!