Calling Oboists! Need a little guidance

Howdy y'all!  

I'm writing an Oboe Quartet (Oboe, Violin, Viola, and 'Cello) and I'm wondering about the upper register of the Oboe.  It's not exactly giving me fits, but I'm  having trouble believing that the instrument is as limited as it seems to be, realistically.

Being a Classicist, I tend to write parts that are intended to be playable on 18th Century instruments.  I'm a string player, but have it on fairly good authority (and actual experience) that the Oboe, circa 1790-1800, was not really capable of playing anything above a D6 (D above high-C) reliably; there was a famous exception in the period, a virtuoso player named Friedrich Ramm (1744-1830) in Mannheim who was capable of playing an F6 (F above high-C), and it was for this player that Mozart wrote his celebrated Oboe Quartet in F, K. 370.  In my own Sinfonia Concertante in C for Oboe, Bassoon, Fortepiano, Violin, 'Cello, and Orchestra, which was performed by the Austin Baroque Orchestra on period instruments (the oboe soloist's instrument was a copy of an original from 1806), I wrote a couple of E6s (E above High-C) that didn't come out well in performance, despite the excellence of the soloist otherwise, and that has made me hesitant to write anything in my other works for the oboe any higher than D6, even in my Oboe Concerto.  

Now I'm wondering if that register above D6 is difficult or unreliable on a modern Oboe.  What do you guys think?  I've had to rethink a couple of passages in this piece I'm working on, and I'd like to know if I'm being a little skittish.  For that matter, if you think my experience with my Sinfonia Concertante was not representative of what a really good player should be able to play, I'd like to know that as well.  Thanks in advance.  

I'll just tag @MJFOBOE in this thread.  Maybe he might be able to help you..

Huge disclaimer: I am not an oboist. 

But I'd imagine it really depends on what you need the oboe to do. My first immediate thoughts go to the intro in Haydn's oboe concerto which, if I remember correctly, holds onto C6 for quite a while. It does go up to D at the end of the first movement, but only on a non-diatonic chord, so that extra brightness is warranted. Even then, it's really short.

I compare that to his contemporaries' concerti, Mozart's, Kozeluch's, and Ferlendis's. On a quick scan, I don't much see anything past C in any of them, though I see some similarities to Haydn's: Kozeluch is also comfortable holding onto C5 for a while and Mozart uses D5 as a brief note in a higher moment. From what I remember for Ferlendis's, it plays it pretty safe, all things considered. I don't have the music, but a quick Google search shows that nothing over C5 is really lingered on.

...or, that's what I would have said until I saw the attached passage. Even still, my gut tells me to say that C5 is where you edge out for longer, full-bodied sounds and things above it like D and E can be used quickly on occasion, but not in the same way as C5, and certainly not commonly. But that's all conjecture; I don't actually know the ins and outs.

Luckily I write modern music, so those suckers have to deal with whatever I give them, hee hee hee!