Kind of how I first heard of them too. Porcupine Tree, Ozric Tentacles, Mandragora, Sun Dial and also Soma (who you didn't mention) and maybe a few others were listed in a Laser's Edge catalog in late 91/early 92. Maybe the Bevis Frond too. Ozric Tentacles were soon signed to a major label (IRS) while PT stayed on Delirium for a while. Maybe because of this I saw a guy wearing an Ozric Tentacles t shirt at a King Crimson concert in 1995 but not a PT shirt. I did see one (just one) PT shirt when I saw King Crimson in late 2001 amid all the Gentle Giant t shirts (weird I know). I didn't actually buy any PT until the late 90s with Stupid Dream though or maybe Lightbulb Sun in early 2001 (I don't remember). 

All those bands are mainstream. I guess we have a different idea of what mainstream is. Most Tool albums have gone double or triple platinum and Arcade Fire won a grammy for best album. As for C&C they have two gold albums (one more than Dream Theater). Maybe PT aren't quite on that level but still have tons of fans and way more than most modern prog bands. Maybe not mainstream but certainly not obscure (or underground) either. PTs popularity goes beyond just prog though which maybe explains why they are way bigger than most prog bands (all but the most mainstream like the ones you mentioned). Opeth are similiar in that like PT they aren't played on the radio but most metal fans know about them and they have a fanbase beyond just prog.

Alex played on FOABP on Anesthetize......

PT might be kinda a popular band now but not very popular IMO. More popular today than ever before as they are turning into a cult band. Most of their album sales have been after say Steven Wilson started releasing solo albums and especially when he started remixing a lot of classic prog and pop albums like XTC, KC, JTull and others. His reputation as a sound engineer has brought PT some additional notice.

If you have not watched the bio on the making of In Absentia, you should. That was supposed to be their breakout album. Which included more accessible songs and more hard rock/metal flavor, they were almost handled with kid gloves by the record company flying them to NYC to record the album with a large budget....Guess what it did not pan out, not at all, from the label point of view it was a flop. In Absentia sold much better years after initial release, I suspect most of their back catalog has seen a resurgence in sales only in the past 10 years.

I think early on (90's) they were an underground band, not till early 2000s did they start getting noticed..........C/C tour was pretty big as it was prime for people who never saw them before, but had been into them since SW solo material and after the last tour in 2009-10 or so with The Incident. I suspect there are a ton of people who never connected SW with PT because his solo material did much better than PT ever did.

Again, the PT catalog has been selling much better but only in the last 10 yrs I bet..

That's true. In Absentia did not chart in most countries when it was released (not at all in UK or US for example). I remember when it came out I did hear "blackest eyes" on the radio and I bought it at cd store near where I was working at the time. It did start to really get the ball rolling for them but it was probably not the overnight success the band and the suits were expecting (as you say). When I saw them in Philly in 2001 for Lightbulb Sun there were maybe 300 people in attendance. The next time I saw them was on the Deadwing tour and was surprised to see about 800 people in attendance (at a 1300 seat venue). It wasn't almost entirely full but pretty full (a little more than 3/4 maybe which is how I arrive at around 800).  The whole point of this thread was me wondering what happened between 2001 and 2005 in particular to make them shoot up so fast? The only thing I can think of is that their music must have somehow been marketed to the non prog crowd and maybe the metal crowd but since I'm not a hardcore metal fan I can't speak to that. I remember talking to a fellow prog fan on the phone named Walter (this was around 2001 or 2002 I think so either right after LS or right after IA). I remember him telling me that if Porcupine Tree played their cards right they could be the next Radiohead. Well, it didn't quite happen but I think Walter was on to something. 

I wouldn't say last 10 years though mainly because they have only put out one album in the last ten years (C/C from 2022). I would say after 2005 and especially after FOABP. When I saw that they played Radio City Music Hall in NYC which seats about 5,500 I knew they had hit the big time (relatively speaking). That's the same venue Pink Floyd played when they premiered DSOTM. And no I don't think PF was quite a household name yet just like PT probably aren't but hey imo that's something.