Since I am the one to whom you refer in the opening post, perhaps I might add a few comments...
First off, I'm certainly not opposed to period instruments; as you note, there are effects achievable on an early forte piano which are very difficult to achieve on a modern piano. Also, the tone and resonance of a forte piano is quite different from that of a modern piano. If one has the resources to own one, and wants to play that music in an "authentic" way, I'd say to go for it.
You enquire, though, about keyboard music which has been reinstrumented. This is an area where I feel rather strongly, oddly: If one is to play music written for harpsichord, it is almost impossible to obtain a similar effect with a piano -- and this applies to almost anything prior to Beethoven. Not to say that a piano reproduction of a Bach or Scarlatti piece isn't wonderful, but it isn't what they would have heard or, possibly, had in mind (now there's a minefield to wander into!). This is also true of the innumerable piano transcriptions of organ music. When one gets into this area, though, one is dealing with some very tricky questions of art and aesthetics; as j_menz says, I do not care for the authentic or else school of thought.
In the interim period, sticking to pianos here for a moment, there is a real question, and to my mind a very valid one, as to what a composer such as Beethoven would have done had he had a modern piano available. This question is a good way to start a pub clearing brawl at a convention of musicians...
With regard to other instruments (I mentioned french horn, as I play that instrument) again one can get into a rather unfortunate fight with the purists vs. the modernists. The purist will argue, correctly, that there is often a different tone quality to period instruments which is not present in modern ones (in the horn, for instance, a period instrument will be darker on its open notes than most modern instruments) and that this is valuable. This is true, so far as it goes. However, there are also some problems with period instruments. Intonation for keyed instruments (flutes, clarinets, oboes/haut bois and the like) is often problematic and requires great skill and attention from the player. Fingerings can be... um... interesting; a pre-Boehm flute is an absolute nightmare for some notes, for instance. The result of that is that there are some passages which are that difficult on a modern instrument which are almost impossible on a period instrument. If you have the skill to do it, fine -- but it's rare. On my own orchestral instrument, valved horns were simply not available until, I think, the mid-1800s. Most earlier music was written so that to the maximum extent possible the only notes called for were open, as the tone quality of the instrument changes radically when it is stopped to sound notes which are not in the harmonic series. Does playing the Mozart horn concerti on a period horn sound authentic? It surely does. Is it more aesthetically pleasing than it would be on a modern horn? To my mind, very doubtful.
But on the whole, it is a fascinating topic... and one where doctrinaire positions are most unwise! Including my own!