So here is my result:
3C120 design, same source, same constraints, two independant directory, compiled twice each for 8.1 and 9.0 (for the second time the design was changed by one line, which pulls different signals to test pins).
Resource usage: 52% in 9.0 and 54% in 8.1.
8.1:
; Analysis & Synthesis ; 00:11:00 ; 1.0 ; 417 MB ; 00:10:52;
; Partition Merge ; 00:00:32 ; 1.0 ; 345 MB ; 00:00:32;
; Fitter ; 00:30:28 ; 1.3 ; 899 MB ; 00:36:48;
; Assembler ; 00:00:18 ; 1.0 ; 514 MB ; 00:00:17;
; TimeQuest Timing Analyzer ; 00:01:36 ; 1.5 ; 617 MB ; 00:02:21;
; Design Assistant ; 00:01:52 ; 1.0 ; 474 MB ; 00:01:51;
; EDA Netlist Writer ; 00:00:46 ; 1.0 ; 468 MB ; 00:00:46;
; Analysis & Synthesis ; 00:02:47 ; 1.0 ; 415 MB ; 00:02:44;
; Partition Merge ; 00:00:33 ; 1.0 ; 346 MB ; 00:00:31;
; Fitter ; 00:30:18 ; 1.3 ; 896 MB ; 00:36:37;
; Assembler ; 00:00:17 ; 1.0 ; 514 MB ; 00:00:17;
; TimeQuest Timing Analyzer ; 00:01:35 ; 1.5 ; 616 MB ; 00:02:20;
; Design Assistant ; 00:01:51 ; 1.0 ; 473 MB ; 00:01:51;
; EDA Netlist Writer ; 00:00:46 ; 1.0 ; 467 MB ; 00:00:46;
; Total ; 01:24:39 ; -- ; -- ; 01:38:33;
9.0
; Analysis & Synthesis ; 00:24:48 ; 1.0 ; 919 MB ; 00:24:31;
; Partition Merge ; 00:00:31 ; 1.0 ; 279 MB ; 00:00:3;
; Fitter ; 00:25:02 ; 1.5 ; 954 MB ; 00:29:45;
; Assembler ; 00:00:18 ; 1.0 ; 517 MB ; 00:00:18;
; TimeQuest Timing Analyzer ; 00:02:22 ; 1.1 ; 613 MB ; 00:02:27;
; Design Assistant ; 00:01:48 ; 1.0 ; 480 MB ; 00:01:47;
; EDA Netlist Writer ; 00:00:42 ; 1.0 ; 480 MB ; 00:00:41;
; Analysis & Synthesis ; 00:06:46 ; 1.0 ; 950 MB ; 00:06:35;
; Partition Merge ; 00:00:34 ; 1.0 ; 283 MB ; 00:00:31;
; Fitter ; 00:24:37 ; 1.5 ; 933 MB ; 00:29:27;
; Assembler ; 00:00:18 ; 1.0 ; 517 MB ; 00:00:18;
; TimeQuest Timing Analyzer ; 00:02:24 ; 1.1 ; 611 MB ; 00:02:29;
; Design Assistant ; 00:01:50 ; 1.0 ; 478 MB ; 00:01:49;
; EDA Netlist Writer ; 00:00:42 ; 1.0 ; 479 MB ; 00:00:41;
; Total ; 01:32:42 ; -- ; -- ; 01:41:50;