I do not know for sure about aftermarket barrels and/or comps. But, I have experience on the reliability (or lack thereof)......
I have a P-18 (their 9MM gun) that I use for 3 Gun. Getting this thing to run has been a very long, laborious process. The first time I tried to shoot it, the disconnector would hang up on the slide and prevent the gun from going back into battery. I sent it back to Para and waited a few months. When it came back it was still too long. I finished the last little bit of fitting on my own. After that, I did:
A trigger job - Pretty painless...... It's around 3# now. I used a Wilson hammer sear for a regular 1911.
Upgraded the extractor - The power claw extractor is absolutely terrible. You can put a standard extractor in as long as you add the special sleeve to fill in the hole where the power claw needed the extra space. Or, Aftec makes one that drops right in. I used a Wilson 9MM specific extractor with the sleeve.
Repolished the feed ramp - I could feel ridges on the factory polish job with my fingernail. Now it looks and feel like a mirror.
Started tuning on the magazines - The factory mag springs have square coils. The coils start small at the follower, get bigger, get smaller in the middle and then get bigger again at the base. This design makes it very prone to coils getting locked in on themselves and making a salt shaker. I changed them to Wolff springs. I also had to spread the feed lips out slightly and polish the inside of the lips so they would work with my 10# recoil spring that I use with my 125PF, 147 grain 3 gun loads. I also added Dawson extensions and they hold 22 rounds.
Now that I've done all of this, the gun is quite reliable and accurate. I still have a little bit of concern with the mags, but the gun itself is very nice and soft shooting.
If I knew what I know now when I purchased the gun, I would have saved up and bought an S_I type of gun. But, I have hung with this one. With that said, as soon as I can come up with the money, I am going to replace it with something nicer. Most likely an SVI or a Benny Hill Fat Free. Until then, I do not think the gun is holding me back. We had a plate rack at ~35 yards at the match I shot this weekend. It only took me 8 shots to clean the rack. That was the least amount of anyone in our squad.
I would not use this for a carry weapon. I trust it enough for a game gun, but not near enough for if the targets were trying to shoot back.