It is not illegal to modify a product/manufacture a "firearm" if you have the appropriate license and register for it if/as required.

The only "illegal" modifications would be if you don't have appropriate licensing, do not register it if required depending on product/where you live, or create some sort of "prohibited device".

The FTP kit will put an M4 over 500fps- if you do not want a "firearm" then you could avoid this by trimming the hammer spring down a half-dozen coils.

The real point of the FTP is to allow less pumps for the same amount of power ouput on a product- i.e. if it takes 8 pumps to get 500fps on an M4 or 760, the FTP would allow getting there in 5 or 6 pumps.

In the case of the M4-177, I would have to assume that if it was brought up to th specs of being a "firearm" it might very well get classified as a "Restricted Firearm" due to its appearance- i.e. simialr to an AR-15 or an M-4, so trimming the hammer spring would be the way to go unless you want to have to license and register it (everywhere in Canada for a Restricted product) and then obtain an Authorization to Transport (ATT) to move it anywhere. You could cut the hammer spring to max it out at 500fps and get there in a reduced number of pumps with the FTP kit.