Splice in with CRISPR/Cas
A paper was published about a week ago in Nature Biotechnology and adresses your question, Maruyama T et al., 2015. I must say I found the authors' strategy extremely clever.

It is not about increasing efficiency by reducing specificity, but simply increasing efficiency (which is your ultimate goal anyway). What the authors did was to inhibit nonhomologous end joining (NHEJ) to promote homology-directed repair (HDR), two DNA repair mechanisms that compete in cells and of course HDR is the mechanism needed for the CRISPR/Cas9 system.

They achieve NHEJ inhibition using the molecule Scr7, a DNA ligase IV inhibitor which in turns perturbes NHEJ.

Using Scr7 they boosted by 3 to 19 fold (depending on the cell line) the insertion of the target gene. Here the graph showing these results

See paper figures

Moreover using 1μM of Scr7 over 24h on DC2.4 cells increased the % of transfected cells from 4.58% to 58.3%, a neat ~13-fold increase. Here their results:

See paper figures

Hopefully this should give you some ideas.