Are you sure that all the intelligence is in the battery itself and there is no voltage/amperage metering in the charger?
We all know that our batteries get most of their charge within the first 45min with the stock charger and then the full charge is complete after about an hour and a half total. That last 45min the stock charger gets noticeably cooler because it is supplying much less power to the battery. Is it the battery doing all the regulation or is it a combination of both the battery and charger?
If it truly is only the battery, then there should be nothing wrong with a Pb-mode charge since the stock charger really is just a "dumb" 12V@4A source. The only speed up you should see by using a higher amp charge would be to reduce that initial 45mins, the second 45mins shouldn't change at all because the battery will be taking less than 4A anyways. This is my linear thinking...
Another member gave a great analogy of charging Lipo batteries like trying to fill a 2L bottle with water that is half full of sponge. To get it full you can add half the bottle (1L) quickly but the other half has to be at the speed that the sponge is able to absorb the water or you will overfill the bottle (or in terms of a battery - cause damage). You can add a bit, then wait, then add a bit and wait, or just slow down the flow of water to equal the rate the sponge absorbs till the bottle is completely full.