I had the same concern about installing downpipes.
However, Mike@N54 let me listen to his setup (catless downpipes with stock exhaust) and took me for a spin and that changed my mind entirely.
If you are retaining the stock exhaust system, then catless downpipes are hardly louder than a stock setup except in two situations:
1) Cold startup will be much louder for about a minute = no big deal.
2) WOT after 4K rpm will sound louder than stock, but still not what I would consider obnoxiously loud. It's actually a really nice sound level that's more aggresive than stock, but still very acceptable in terms of loudness.
While cruising around town, there is hardly any difference from stock setup sound levels at all.
It's a nice balance between noise and performance.