The original Springfield Hellcat is one of the original micro-compact double-stack 9mms. Coming out directly on the heels of the original Sig P365, Springfield seemed to directly challenge Sig with a 1.5-stack style magazine, offering the end users a hefty 11+1 round capacity. This capacity seemed magical at the time as the original Glock 43, one of the more popular micro-compacts of that time, only carried 6+1 rounds of 9mm. One might question the reason you might carry a Glock 43 over a revolver when a Glock 19 carried 15+1.
It should be noted that the overall platform became even more potent when the Hellcat Pro came out with the same ultra-concealable slim profile, but with a 15+1 capacity. I carried that version for over 1 year as my primary EDC. The Pro comes with a native optics mounting area that, unlike the P365, was not compatible with the Holosun K design for pistol red dots. This meant adding even more parts to the gun and hoping everything worked out.
Regarding performance—both firearms are capable of great accuracy within 15 yards with enough practice. The original HC was on the snappy side, but featured an incredible grip texture that was gentle on your skin but locked into your grip with the right pressure applied. The Hellcat Pro did have a certain overall low-axis punch into the center of your palm vs. more of a flip, but with a longer grip and barrel you encounter less punishment from 9mm.
Upgrades? - I spent the first year or 2 trying parts from multiple brands and I've found some options I personally like:
Hellcat (Original/OSP)
The first thing you might invest in is a reliable mag extension or extended magazine as the 11 or 13-round mag is great, but for the range or preference, 15+1 is always better and does not create tremendous issues while carrying. I would get OEM 15-Round Mags.
I would personally recommend going with the Holosun SCS Carry. I've used that optic for over 1 year and I think it might be best in class. The SCS Carry is small, enclosed to block fog, lint, and other elements while also having a solid auto-adjust brightness system. You will need an Optics Plate to use that optic, but it is worth it.
Hellcat Pro
I have tried a few different things with the Hellcat Pro. The best upgrade I made was getting the OEM 17-round mags. Easily the best choice for more fun at the range.
I ran the SMSC optic for multiple years and I felt it was lacking in overall features, but it held up for the entire duration and the battery lasted for about 2 years without issue. I would tell you to opt for the SCS Carry with the Optics Plate.
The internal upgrades we tried and enjoyed for a long time came from Apex Tactical. We tried the full kit. We eventually tested the Tyrant CNC flat profile trigger, but given the overall quality and included parts with the Apex, I would recommend that option. I'll list them below.
Apex Tactical
Tyrant CNC Parts