Snow Leopard 64 bit kernel on Mac Pro - Personal Experience.

Question

Curiosity got the better of me so I decided to install Snow Leopard on clean external USB drive (this is a rather slow drive, I believe it is only 4500 RPM), but good enough to test things out.
I formatted the drive before installation in Leopard (HFS+ Journaled, GUID partition), booted off the install DVD and proceeded to install on this external disk. Installation took about 50 min (this is not upgrade but clean install).
After the install was finished, I manually copied over some applications from my Leopard install (together with their application settings and preferences files in ~/Library and /Library). All of them functioned properly.
I am happy to report that Photoshop CS4, Nikon Capture NX 1.3, Sling Player, MacVIm (without ATSUI text renderer), iDefrag, Toast Titanium 9, Transmission etc all worked with no issues.
The color profile for my 30'' ACD screen looked a little too bright and less contrasty (this is gamma 2.2 profile I created for Leopard some time ago), but it is rendered differently in Snow Leopard. Which brings me to another point. My ancient Monaco Optix XR screen calibration software (with USB spectrometer) works with Snow Leopard as well. It does require Rosetta though.
Next thing I tried booting in 64 bit kernel. That worked fine, but now I wanted to add my printers. HP LaserJet 1300 was recognized instantly and added for me, so I guess Apple already provides 64 bit driver for it.
Next is the Epson Stylus Photo R2880. It was recognized (as in type and model) but no driver was available for it. However, Epson has just recently released the driver for it. The question was is it 64 bit as well. As it turns out the installer for the drive requires Rosetta, which wasn't very encouraging, but once installed the driver works just fine with 64 bit kernel. So I guess they do provide 64 bit driver as well.
As a final step I did X bench performance comparison:
This is the result for Snow Leopard
Results 209.29
System Info
Xbench Version 1.3
System Version 10.6 (10A432)
Physical RAM 32768 MB
Model MacPro3,1
Drive Type WD 5000BEV External
CPU Test 199.19
GCD Loop 329.53 17.37 Mops/sec
Floating Point Basic 159.06 3.78 Gflop/sec
vecLib FFT 131.91 4.35 Gflop/sec
Floating Point Library 314.56 54.78 Mops/sec
Thread Test 841.97
Computation 1155.73 23.41 Mops/sec, 4 threads
Lock Contention 662.20 28.49 Mlocks/sec, 4 threads
Memory Test 213.36
System 336.79
Allocate 606.04 2.23 Malloc/sec
Fill 247.60 12038.67 MB/sec
Copy 310.68 6416.90 MB/sec
Stream 156.13
Copy 157.24 3247.78 MB/sec
Scale 157.88 3261.67 MB/sec
Add 155.20 3306.02 MB/sec
Triad 154.28 3300.33 MB/sec
Quartz Graphics Test 216.11
Line 192.80 12.84 Klines/sec [50% alpha]
Rectangle 237.36 70.87 Krects/sec [50% alpha]
Circle 204.41 16.66 Kcircles/sec [50% alpha]
Bezier 229.12 5.78 Kbeziers/sec [50% alpha]
Text 223.22 13.96 Kchars/sec
OpenGL Graphics Test 96.71
Spinning Squares 96.71 122.69 frames/sec
User Interface Test 356.31
Elements 356.31 1.64 Krefresh/sec
And here is the equivalent test for Leopard 10.5.8
Results 252.74
System Info
Xbench Version 1.3
System Version 10.5.8 (9L30)
Physical RAM 32768 MB
Model MacPro3,1
Drive Type WDC WD3200AAJS-41VWA0
CPU Test 190.72
GCD Loop 327.96 17.29 Mops/sec
Floating Point Basic 160.63 3.82 Gflop/sec
vecLib FFT 134.16 4.43 Gflop/sec
Floating Point Library 235.57 41.02 Mops/sec
Thread Test 830.59
Computation 1124.19 22.77 Mops/sec, 4 threads
Lock Contention 658.59 28.33 Mlocks/sec, 4 threads
Memory Test 202.30
System 284.32
Allocate 314.73 1.16 Malloc/sec
Fill 243.49 11838.90 MB/sec
Copy 306.07 6321.67 MB/sec
Stream 157.00
Copy 160.48 3314.67 MB/sec
Scale 157.71 3258.27 MB/sec
Add 155.89 3320.85 MB/sec
Triad 154.07 3296.01 MB/sec
Quartz Graphics Test 243.39
Line 219.20 14.59 Klines/sec [50% alpha]
Rectangle 295.86 88.33 Krects/sec [50% alpha]
Circle 240.56 19.61 Kcircles/sec [50% alpha]
Bezier 235.66 5.94 Kbeziers/sec [50% alpha]
Text 238.06 14.89 Kchars/sec
OpenGL Graphics Test 185.37
Spinning Squares 185.37 235.15 frames/sec
User Interface Test 351.35
Elements 351.35 1.61 Krefresh/sec
As you can see the overall system performance has degraded quite a bit. Snow Leopard performance is only about 82% of Leopard performance.
Close inspection shows that some things are actually faster in Snow Leopard, e.g. CPU has higher score, Thread test has higher score, Memory test is faster. However, quartz test is slower, and Open GL text test is 2 times slower.
This is quite a degradation in performance, and I'm hoping it will be addressed in GPU driver upgrades.
Also, I also noted the dreaded choppy animations, esp for dashboard, which is so choppy it is embarrassing. Like I said, this will probably go away with better driver.
I hope someone else finds this useful.

Answer 1

I plugged the external drive with Snow Leopard (it's set to always boot in 64 bit kernel), and booted up iMac with it. It started fine, but it decided to use 32 bit kernel anyway. This is iMac version 7.1 with 64 bit EFI and 4 GB of RAM. It should technically be possible to boot 64 bit kernel, but doesn't work. I won't even try my Aluminum Macbook .