I know that AHU and ALH auto tranny TDI starters work on manual IDI cars. But do they spin the engine a lot faster than the IDI starter?
Depends, What year IDI starter?

If it is the straight drive mk1 units, then yes. Substantially faster. But I haven't noticed much extra speed if any over the already gear reduced mk2-3 diesel starters. The real advantage is they are plentiful, cheap, strong and reliable. If you have slow cranking speed now on your 85, you have an issue with your starting system. That starter and system when in proper order should have no issue really turning that 1.6.
Thanks for the reply. I was just wondering if they had a gear reduction that spun them faster than the OE for the 85' 1.6. And since it doesnt make much difference than I dont have to look for anything special if I ever go to change it. Its not spinning slow or anything. Just wanted to know for future reference.
So does that mean it will work in a 81 Caddy with a 020?
I have an '01 TDI auto starter in my '79 rabbit w/ 020. I was mildly disappointed when I installed it as I couldn't tell that it spun any faster than the original starter.
Well maybe as a consolation prize, it's lighter.
The gear reduction setup has the advantage of preserving the motor itself - it is much easier on the windings for an electric motor to spin faster than slower. All other factors being equal a slow turning electric motor generates a lot of heat which in turn degrades its windings.
The TDI auto version also has the advantage of not needing a bushing in the trans to get its job done. One less thing to wear out.
Yes, those are all good points and the reasons why I was only mildly disappointed.