As Ususal, It Depends...
 

Intercoolers are simple and reliable, but they have a pretty difficult rate of diminishing returns to get past if you are talking a highly fueled truck from around the mid 90's era or later. That's when they all started getting fairly good coolers at the factory. Aftermarket units will give modest to moderate gains at higher to higher cost. The biggest top end units are quite pricey.

The 1st Gen Dodges had fairly crappy, small intercoolers - the last years (91.5-93) that even had them at all, that is. I installed a 2000 Powerstroke intercooler in my 1st Gen to alleviate that issue and saw and average of around 50 degrees improvement across the board. But that's all there is to it. It's there, does it's job and never changes. And it's not enough on the high end.

You really need the EGT control when towing heavy, racing, etc. especially if the truck is running higher fuel levels than the turbo can effectively support. It is very easy to over fuel with the mods like bigger injectors and tuners out there. It's happening under heavy load you don't see but for maybe 1% of the driving time, but the results can be catastrophic. That's where water meth comes in.

I see around 100 degrees difference on the top end after the w/m kicks in with my trucks. YMMV. Unless you drive with your foot on the floor most of the time, w/m is the best bang for your EGT control buck.

If you are running all out with alot of fuel all the time you need a huge $2k intercooler and twins.

Fantastic answer. It even touched on things I had not considered, such as the all too common undersized turbo.