If you plan on doing ANY engine mods to your car, a FlashPro is an absolute MUST. Seriously...you NEED to have it. It lets you literally change EVERYTHING in the ECU...fuel maps, ignition maps, V-Tec and VTC engagements, speed limiter, gear-ratios...I could go on and on. Seriously...
I got it for my (at the time) BONE stock Si. I found a baseman (I believe it's an older Vit Viper map...?) and it literally changed the entire car. No more rev hang. My MPG bumped up 2-4 MPG. The car is faster. Again...I could go on.
Yes...it's really expensive... But 1)if you plan on modifying your car for power/speed, it's more than worth it and 2)you'll need a tune to make those bolt-ons worth it. You can send your specs to e-tuners (guys who literally tune online..), and they will send you a 'map' of your tune. You open it up in FlashPro, and download it to your ECU and bam. You can now utilize your expensive bolt-ons.
I found mine for $500, used (just be careful going this route...make sure the FP is UNLOCKED from the current owners car). Brand new I believe they're $695...?