As someone who rides a almost no-sound Zero S, I can tell you that having loud pipes works against you as much as for you. A keen pair of eyes and ears and riding skill has no replacement at any rate. What cops will tell you is that often older drivers become disoriented/confused by loud noises. Sure they hear the siren or see the lights, but they do not know where they are coming from and often do EXACTLY THE WRONG THING, ie. move over right into the cop's path. At any rate, I've put 3,000 miles on electric motorcycles riding fairly aggressively and have never had a issue, or a close call. Remember too, many drivers are on phones, have loud radios or are otherwise distracted. Observing their behavior is by far the most important safety measure...especially knowing when someone might pull out into your path, change lanes quickly on you, or turn left into your path. Those folks are not going to be aware of much in any case if they can't even see the friggin motorcycle.
One of the other Grom Forums has a first ride review from a British guy of an MSX125 who says the horn is loud, so you may not need anything louder. For some reason I can't post that review, but you can find it I'm sure. He's 5'11" and says he almost can't see the bike and feels perched on top of it. He didn't really say that in a negative way necessarily (as some nameless individual described it as a frog on a lily pad). Anyway he gave it an 11 out of 10 and he is an actual rider. Didn't like the front suspension...too soft and didn't like the seat...too hard. Anyway read the user review for yourself if you have time. I'm not sure we will get exactly the same bike. At any rate the owner's manual from GB talks about the oil dipstick, but on a couple of photos of the Grom it looks like there is a sight glass. I wouldn't expect any major changes though....oops, I digress, again. The horn is loud.
I have spent hours trying to interpret those dang dipsticks. A sight glass is so much better...plus you can see the color of the oil. The good news is, it takes one quart of oil, so I assume that you will put in just short of a quart when you change oil. I can trust myself to do that and pretty much forget about the dipstick. I'll just buy my oil in quart sizes. Usually I use a big jug of synthetic, don't measure how much I put in, then have to keep measuring. Think I will use regular dino 10W30 T4 racing oil. Yes on the assuming they don't see you at least until you know they do by their actions, where they are looking and how they are handling the car. I do a lot of bicycle riding (e-bikes) and some older people around here do not think they can get around you without crossing the center line, so they just crowd you off the road instead. I am very leery of old people in big old cars.
Shoot, people don't see me in my Miata, I constantly have to dodge people trying to cut over into my lane or merging all crazy. I've already come to terms that being on a bike is a whole new ball game of not being seen in traffic.
Never assume other drivers can see you. Both my truck and my Ruckus were straight piped, that in addition to being alert to other drivers is the reason I've never been in an accident. So many times I've seen people shoulder check to lane change into me, and all I've had to do is crack the throttle and they stay put.