Yep. if already cruising at 20 mph, 100w of pedaling gets you to about 21 mph.
RE the PAS lurch, that's why PAS bikes need those brake cut off levers, which I typically omit on a throttle only bike.
I finally rode good PAS at interbike, It was the bosch mid drive on a Scott MTB. I don't think it was torque based, just well tuned. Many of the other bikes I rode with mid drives had lurchy PAS, at least if you had the power level high they did. The way I like to ride, I want that lurch when I start up from a stop sign. I want to stay in the highest gear the whole ride. But I want a throttle to control when that lurch happens. That's what I did with those crappy PAS bikes I test rode at interbike. Most had throttle and PAS, so I just hit the throttle, then once going started pedaling, then continued in PAS mode. That works good! 8) But I did learn fast, to keep those brakes on till I wanted to go.
Throttle and go IS fun. That's why one of my dirt bikes, and one of my cargo bikes are 2000w, and quite capable of a motorcycle or moped like ride. Fast and fun. But both can still be ridden pedal style, pick a gear, hit a comfy cadence in that gear, then throttle till your effort leaves you sweat free but still feeling pressure on the pedals.