Yes, IPSec overhead consumes bandwidth. Can much be done about it? Yes and no. If you configuration is suboptimal, you'll consume more bandwidth but even an optimal configuration is likely to add nearly 100 bytes per packet, which really adds a lot of overhead to small packets.
Can you limit its bandwidth consumption using QoS? That depends. QoS won't have any impact on the overhead per packet, but you might be able to limit VPN bandwidth relative to other non-VPN traffic. (Actually, if you're going to go down that road, you would be better off limiting bandwidth of some applications relative to other applications regardless of whether the traffic was VPN or not. For example, you could have VoIP traffic in both VPN or outside it, and such traffic, generally, shouldn't be bandwidth limited by QoS [beyond call control - i.e. number of calls allowed/accepted].)
