These days, companies use the term "long throw" for anything over 5mm one way. Typical cheap woofers will have Xmax of 3mm, even less. But it's not like 5mm is gargantuan or even uncommon. Most decent woofers available from SEAS, Peerless, Scanspeak, SB Acoustics, have Xmax values of at least 5mm on their drivers of that size.
It's great that small drivers can deliver decent bass levels and makes small monitors attractive, especially for someone who doesn't require concert level SPLs. But attempting to get 100dB at 40Hz from a single 5" woofer is just begging for massive amounts of distortion. High levels of bass energy are best left to drivers designed to produce it.