I remember the name XTC, and I remember "Senses Working Overtime" from the heyday of MTV, but I probably haven't heard the song in 35 years and wouldn't have connected it as an XTC song if I hadn't just googled it.
Anyway, interesting discussion about Styx.
When I first got into the band, I naturally gravitated to JY because he was the "rocker" of the group, and "Miss America" was (and still is) catchy. "Put Me On" also kicks ass (although that's credited to DDY, Shaw and Young, so who knows who wrote what). My favorite all time Styx song -- "Queen of Spades" -- is also a Young/DDY composition. But that's pretty much it in terms of JY contributions -- either vocally or songwriting. My list of five star Styx songs would be the following:
Suite Madame Blue DDY
Put Me On DDY/Shaw/Young
Crystal Ball Shaw
The Grand Illuision DDY
Fooling Yourself Shaw
Come Sail Away DDY
Man in the Wilderness Shaw
Castle Walls DDY
Sing for the Day Shaw
Blue Collar Man Shaw
Queen of Spades Young/DDY
Renegade Shaw
Pieces of Eight DDY
The Best of Times DDY
One with Everything Burtnik/Gowan/Shaw/Sucherman/Young
JY was/is sort of the Ringo Starr of the group, and a lot of his songs are sort of comic relief.
I don't know much about who contributed what beyond the writing credits as shown on the album (the only thing beyond that I've ever heard was the story about DDY suggesting that "Renegade" be made into a rocker), but I don't think anyone would seriously disagree that Styx wouldn't have succeeded without DDY. It would be interesting to see what might have been if JC hadn't left the band, but I doubt it would have been anything significant.
The other interesting thing to think about is the Panozzo brothers. Neither had a single songwriting credit (I don't think), which means that, when Styx stopped selling albums and wasn't playing live, they had no income (unless the band organization was set up to distribute songwriting income).