I've seen a lot of tweak guides that all claim to unlock your FPS but after following them I have not seen any improvement. The smoothed FPS still locks to 62fps and the Variable FPS still locks to max 150 fps.

I have a solution that actually lets you go above 150 fps.
The game runs on Unreal Engine 3 and there are no gamebreaking issues above 150 FPS that I have encountered.*

*In the main menu the transition animations are faster above 150fps, this however does not break anything and could be preferable to some.

Smooth gameplay is not achieved by increasing your maximum frame rate, it is achieved by stabilizing your frametimes.

Having the game jump between very large differences in FPS can feel jarring. Having a maximum FPS of 144 and then suddenly dropping down to 80 FPS will not feel good even if its for a short time because your frame time between those images are so different (6.9 ms to 12.5 ms between each image).

Minimizing this difference will make the game feel smoother than wandering between a very high maximum FPS and a minimum FPS.

With Frame Rate Smoothing, the application is determining what range is acceptable for Frame Rate wandering, so it can cap your Frame Rate between Min and Max allowable values and achieve stable frame times for you. How it does this varies on a game by game basis. We can use the commands for Smoothing to allow a higher framerate.

The config files can be found in:
C:\Users\ USER HERE \Documents\My Games\KillingFloor2\KFGame\Config

Open the KFEngine.ini and KFGame.ini files in Notepad++ or a similar program. There are two rows we need to edit to increase the limit.

There are three commands that relate to your framerate limit.
bSmoothFrameRate=TRUE
MinSmoothedFrameRate=22 - your minimum fps in the range.
MaxSmoothedFrameRate=62 - your maximum fps in the range.

bSmoothFrameRate - Keep this TRUE for this guide to work. All other guides I've seen set this to FALSE, which disables the FPS Smoothing option. Setting it to FALSE will lock the game to the default 22-62 fps range (or the 150 fps Variable FPS limit).

MaxSmoothedFrameRate - The value this command has is what your game will limit its FPS to. It can be set to any number between 30 and 999.


First hit on your search should look like this:
bSmoothFrameRate=TRUE
MinSmoothedFrameRate=22
MaxSmoothedFrameRate=62

Change "MaxSmoothedFrameRate=62" to whatever FPS you want as your maximum FPS.

Example:
bSmoothFrameRate=TRUE
MinSmoothedFrameRate=22
MaxSmoothedFrameRate=999
*This results in a completely unlocked FPS range*

There are two "MaxSmoothedFrameRate" commandlines in KFEngine.ini. Make sure you are editing the commands under the [Engine.Engine] tag. There is no need to touch anything in the [UnrealEd.EditorEngine] tag.

Search for "MaxSmoothedFrameRate="
You can find it in the [KFGame.KFGameEngine] tag.
Set it to the same maximum as you did in KFEngine.ini.

Instead of getting your absolute maximum FPS all the time, you might consider limiting your FPS.
This can get you a smoother experience playing the game. There are a few different ways you can go about this:

If you want the smoothest experience you can have:
Use a program like Fraps and Frafs to get your Average FPS and limit it to that.

If your PC has a weak CPU that can't keep up with your GPU: limit it to something the CPU can handle. This will take some trial and error, a good start is the default 62 fps.

Get a screen-tear free game:
Set the max fps limit 2 frames lower than your screens refresh rate while using Gsync or Freesync in combination with Vsync enabled in your Nvidia/AMD control panel. (Must have a display with adaptive sync capabilities)

There is a way to increase your FOV range.
Open KFEngine.ini
Search for "AspectRatioAxisConstraint=AspectRatio_MaintainXFOV"
Change the X to Y.
"AspectRatioAxisConstraint=AspectRatio_MaintainYFOV"

Save the file. You now have increased FOV on the whole slider.

This is a list of guides and sources that I have used to aquire the knowledge in this guide.

[Guide] Tweaks and Improvements - by CryCel

[Guide] Enable sound on focus loss (alt-tab etc)

[Guide] More FPS on ANY Computer | KF2 By Peppr

What exactly does "Smooth Frame Rate" do? - Answer by Lovecraft_K[answers.unrealengine.com]

Screen tearing - Wikipedia[en.wikipedia.org]