The change from 35 / 19 to 41 / 18 means you will have lost your old lowest gear and your new 1st gear will be the same as your old 2nd gear.
And gained a significantly higher top gear (23%) higher. This in turn will increase the max assist speed from 15mph to around 18.5mph.
Changing the motor sprocket from 9 tooth to 11 tooth will raise the cadence (pedal revolutions) at which assist stops from around 58RPM to 71RPM. This will further increase max assist speed up to around 22.5mph. There is a power taper towards the maximum assisted cadence so in practise you're likely to find the assist available is not a great help towards its new top assisted speed.
You should still feel significant assist at around 18/19mph (about 60RPM cadence) though.
The other side is that if you actually use the assist at higher speeds your range will reduce to at best 2/3 and more likely 1/2. That may well not be a problem for you with an 18ah battery.
The other downside is that it makes it illegal unless registered, licensed, insured etc. Plenty of people seem willing to take that risk and that debate has been had several times on here. You need to at least be aware of it and choose accordingly.
Personally I much prefer the feel of the assistance being available at a higher, and for me, more normal cadence, but I also prefer to err on the side of caution legally. It is possible to change the gearing through chainwheel and sprockets to achieve that and still have the larger motor sprocket providing assist at higher cadence.
