I've driven for a couple of months without undertray or front wheel arch liners. Yeah you'll get a few more pings from gravel and so on, which is annoying, but otherwise no significant difference. The reason they were off is I had a new aftermarket bumper fitted and it didn't have any points to attach them. The arch liners attach to the undertray, so when I put them on (without the tray) I just got creative with cable ties for that bit. Fine so far.