I don't have specific numbers, but there will be some drop in performance, albeit a slight one.
There's a blog post: Performance impacts of TrueCrypt on a system partition that describes the general performance of complete disk encryption on a system partition and how that can affect the users perceived performance. It seems to indicate that CPU takes a bigger performance hit than the hard disk:
For me the critical question was what kind of overhead does encrypting your hard drive have on the performance of the system as a whole. To try and measure this I used HDTune to measure drive performance before and after encryption with TrueCrypt. Before encryption I saw an average transfer rate of about 47MB/s with my laptop's SATA drive in AHCI mode. Max was about 59.9MB/s and a burst rate of 85.8MB/s with an average CPU usage of 4.1% throughout the test.HD Performance before Truecrypt
After encryption I saw an average transfer rate of 46.9MB/s, peak of 59.7 MB/s, burst of 62.9MB/s and an average CPU usage of 26.7%. I didn't expect it, but that's where I saw my hit. It makes sense when you think about it - the encryption/decryption of the data generally doesn't result in reading/writing significantly more data (encryption and decryption is done at the block or sector level not at the entire file level) so you don't see substantially reduced disk performance.
Tom's Hardware also has a good article, "Protect Your Data With Encryption", that details the performance implications of using TrueCrypt on a complete volume.
Also, see Scott Gu's blog post regarding the speed of the physical hard disk (i.e. the RPM) (Tip/Trick: Hard Drive Speed and Visual Studio Performance) which can make a big difference to Visual Studio's performance whether encryption is employed or not.