What is the highest capacity / Best battery replacement?

The problem you are facing is the limits of the volume of the space and the chemistry of the battery. When you got your system, the chemistry was still being improved so today we have maxed out the tuning of it and the physical limits of the space (volume) sets what you can put in. This is what the iFixit battery offers.

• Watt Hours: 77.5 Wh
• Voltage: 10.95 V

And this is what your system needs MacBook Pro 15" Unibody (Mid 2009-Mid 2010) Battery.

Yes, you might find a few people advertising batteries which are slightly better lets say 78 Wh (.5 Wh more) these are not any different that what iFixit offers just someone rounding up to the full digit so theirs can be sold as being better! To get a few dollars more out of your pocket!

If power on the road is that important then you should alter your system by pulling the HDD and putting in a bigger SSD. Even still it won’t buy you that much more juice on the road. SSD’s use a lot less power than HDD’s. And you want to make sure you don’t over fill the SSD as that will fire off Wear Leveling which will chew your battery.

There where a few companies making external power packs for the MacBook Pro’s. Here’s one unit Hyper Shop - MagSafe Battery Pack. At one point we had 100 of these for the engineering teams when they went out to the field where there was no power.