You control power through the Cuts entries. The S-value is the total available range.
Not sure what you mean by “I have changed it in the cut tab and not changes are made while in laser mode” - Do you mean you aren’t seeing a beam, or it’s not firing with any power?
For 3018’s, the only way to make them faster is to increase the allowed speed in the firmware settings ($110 and $111), but because of their very high micro-stepping rate (usually 1600 or 800) they’re limited by the 8 bit controller.
8-bit GRBL can only send step pulses at a maximum of 30,000/sec. If you have 1600 steps/mm, that means a max of 18.75 mm/sec (1125 mm/min). If the steps are at 800 steps/mm, you can get up to 37.5mm/sec (2250 mm/min). You would need to reduce the micro step settings on the step drivers to go any faster.