I'm into an HIP phase right now. I don't reject non period instrument recordings, however, as so many of them are really excellent. It's just that I grew up with those and the sound of the period instruments is fresher for me. The balance between the various instruments is also different than with modern instruments so that the structure shows different elements in greater relief. One of the problems with using a modern instrument orchestra for Bach is that Bach didn't score for more than one or two instruments per part and the baroque instruments did not have the sonority of modern constructed instruments. So many of the prized Stradivarius and Amati violins from the baroque period have been taken apart, cut down, built up, and given modern style interior structures so that their sound would be better suited for the late 19th century orchestra. Most modern orchestra performances of any of Bach's concertos usually include a harpsichord for the figured bass, but you can barely make out the sound of the instrument. In performance with authentic instruments, the harpsichord is never lost. In fact, there are many recordings of the violin concertos using modern orchestras which I'm sure you would find very pleasing. One of these, with Isaac Stern and Itzhak Perlman is available in an expanded edition from Sony Classical. That is certainly not a recording where you will feel the strings have been slighted.
In fact, most of the recordings of the harpsichord concertos using piano also include a harpsichord for the continuo parts. Bach performed on the modern piano can be extraordinarily beautiful, depending on who is doing what, but I prefer the piano for solo instrumental music. For the concertos I think you or anyone is better served with a smaller ensemble and a harpsichord. FWIW, there is no one correct way of performing music. There is only one incorrect way: badly.
If you still need more powerful strings for the harpsichord concertos, perhaps consider the harpsichord concertos on piano. The best ones that I have heard are by Murray Perahia. Angela Hewitt also has a fine set of the harpsichord concertos on piano but Hewitt tends to be very rhythmically driven, so there can be an absence of color in her work on piano.
I still think the best choice for the harpsichord concertos has to be the Mortensen recordings with Concerto Copenagen right now. They are less expensive and easier to find than the Hogwood, and this set has far superior sound quality to even the Pinnock. I haven't read a review of these recordings that isn't written in superlatives, and the second volume was even more favorably received than the first. What makes it really nice is that they can be had at very good prices.
If you decide to go for the Murray Perahia Bach, then I suggest this set which also includes the English Suites and Goldberg Variations (really top notch) as well as the concertos. Hopefully it's still available at the low prices I bought it for.
