In theory, QoS should encompass your concern of using multiple paths to provide service as you specify. In practice, your issues of load balancing (using PBR) and QoS were treated separately. Practical QoS often dealt with congestion management.
The latest IOS (e.g. 12.4T) provides a new technology, PfR (performance routing), which, I believe, if combined with routine QoS would do about everything you desire, at least for outbound traffic. However, I haven't used the PfR technolgy to the extent it appears you would require.
If it weren't for your NAT, I might be able to provide an example on using a simple version of PfR (or OER - optimize edge routing - an earlier version of PfR) and QoS.
However, even if we had everything working correctly for outbound traffic, there's still an inbound traffic issue. Managing inbound, generally, doesn't work very well, so you might find half a solution isn't any solution.
Yet, all might not be lost if your DSL is ADSL and likewise your fiber is asymmetric bandwidth too. In that case, assuming your outbound provides much less bandwidth than inbound, using advanced techniques and/or technologies outbound alone might be a benefit.
You can also start with a very simple QoS implementation. For instance, assuming the fiber connection provides 5 Mbps outbound but you have a 100 Mbps Ethernet interface, even something as simple as a traffic shaper configured for 5 Mbps (outbound) might make a noticable difference.
