Here's a little how-to so that everyone can try this. Note: The offsets are for the latest v1.0.0.5 of Sonic Generations, make sure that's what you have.
1. back up your SonicGenerations.exe
2. open SonicGenerations.exe in a hex editor (I'll be using HxD as an example)
3. go to offset 12E1094 and make a selection of length 7C hex (Ctrl+E, then verify values in status bar)
4. copy selected bytes (Ctrl+C)
5. go to offset 12E1164 and make a selection of length 7C hex (Ctrl+E again)
6. paste (Ctrl+V)
7. save
Congratulations, now the game uses the Unleashed renderer.
To get light shafts, you have to do some more work, namely edit the SceneEffect.prm.xml in the #xxx.ar.00 files (eg. #ghz200.ar.00). Find the <LightShaft> section and change "<ms_IsEnable>false</ms_IsEnable>" to "<ms_IsEnable>true</ms_IsEnable>".
You can use MainMemory's editor to unpack and repack .ar files, or you can do this simple change with a hex editor. Be aware that the value has to be "true", not "true " (no leading or trailing spaces). So if you use a hex editor, I suggest replacing "<ms_IsEnable>false" with " <ms_IsEnable>true" (leading space before the xml tag, not inside the value).
For directional shadows, change ms_IsEnableVerticalShadow to false in the same way.
Now you can make your own little mod with the adjusted #xxx.ar.00 files. Here's a suitable mod.ini:
[Main]
IncludeDirCount=1
IncludeDir0="."
[Desc]
Title="Unleashed Shaders"
Description="Swaps shader_r and shader_s files"
Version="1.0"
Date="2012-08-02"
Author="Korama"
[CPKs]
disk\bb3.cpk="bb3.ini"
And bb3.ini needs to contain this:
[Main]
CommandCount=1
Command0=Swap:SwapShaders
[swapShaders]
; swap the Generations shaders with the Unleashed shaders, discovered by Lobotomy
shader_r.arl = shader_s.arl
shader_r.ar.00 = shader_s.ar.00
shader_r_add.arl = shader_s_add.arl
shader_r_add.ar.00 = shader_s_add.ar.00
shader_r_add.ar.01 = shader_s_add.ar.01
shader_r_add.ar.02 = shader_s_add.ar.02
Place these mod.ini and bb3.ini files in a new folder, make "bb" and "bb2" subfolders and place your modified #xxx.ar.00 files in those.
Then link this mod to Sonic Generations by using SonicGMI or do it manually by editing cpkredir's ModsDB.ini.