I realize that it's been a week since this thread was active, but I think this weapon needs more discussion. I may be biased, as it's my current favorite.
Version differences:
I prefer the Forge model, mostly due to the sub weapon. I'm not very good at using Splat Bombs, and the Pro needs all the ink it can get, so I prefer to stick with point sensors. I am interested in what people have to say about Splat Bombs and their ink consumption, specifically when combined with the Pro. Both special options are rather good in my experience, I don't have too much of a preference there.
Matchups:
The Pro's long range and pretty quick kills at any range seem to give it rather good matchups all around. The best matchups I see are against weapons such as Aerosprays and Splat Rollers, as they struggle with your massive range advantage as long as you're positioned properly. I find that due to its massive ink consumption, however, it's best to fight 1-on-1 and in your own ink, even more so than most other weapons, as the Pro cannot afford to waste ink in a firefight spreading turf or fighting multiple enemies.
Gear:
I would certainly optimize gear for the Pro's high ink consumption, by prioritizing Ink Savers and Ink Recovery Up. I always like to stick at least one defense boost on any set to mess with enemy .52 Gals who don't use Damage Ups. Proper positioning is quite useful, so I look for Swim Speed Up as well.
Range and DPS:
One of the Pro's greatest assets, it outranges a large amount of other guns. Abuse of this range appears to be the key to doing well with this gun; otherwise, you might as well use a Splattershot. Its DPS is on the lower-mid end of things, from what I see in the data people have drawn up, but it's still quite effective at all ranges. It isn't so bad as to be worth worrying about in most firefights.
Changes:
I'd like to see how a Disruptor would work on a Pro set, as slowing people down always helps to confirm a splat, especially at range. In addition, I think I'd like an Echolocator. My play with the .96 Gal has shown me how useful the instant inkfill with no drawback can be on a gun with high consumption. Having the radar is always fun, too.