Seems like 4 seconds improvement, from last year (Race to Current Test), including around 1 to 1.5 seconds from the new surface itself, plus the advantage of cooler weather (which has generally accounted for 1 second from winter to summer in quali and around 4 tenths in race). That leaves with just 2 - 2.5 seconds performance improvement from the car in the last 12 months!
Hulk qualified with a time of 1m21.397 and started his race with 1m28.858 in last year's race. That is 7.4 seconds difference. If we extrapolate the same with his current starting stint of 1m24.7, he should theoritically qualify today with a time of 1m17.3xx with Softs!
2018 Winter Testing
2017 Race
27 N. HULKENBERG
LAP TIME LAP TIME
1 14:04:46
2 1:29.539
3 1:28.858
4 1:28.548
5 1:28.845
6 1:28.883
7 1:28.698
8 1:28.478
9 1:28.360
10 1:28.365
11 1:28.628
12 1:28.635
13 1:28.655
14 1:28.390
15 P 1:31.631
16 1:46.316
17 1:27.617
18 1:27.951
19 1:27.598
20 1:27.547
21 1:27.760
22 1:27.814
23 1:27.613
24 1:27.792
25 1:27.652
26 1:27.519
27 1:27.662
28 1:27.807
29 1:27.739
30 1:27.806
31 1:27.730
32 1:28.026
33 P 1:36.829
34 2:18.122
35 1:59.302
36 1:28.309
37 1:26.880
38 1:27.260
39 1:27.325
40 1:27.482
41 1:27.100
42 1:29.356
43 1:27.271
44 1:26.908
45 1:26.839
46 1:26.922
47 1:27.093
48 1:27.006
49 1:27.127
50 1:27.117
51 1:27.020
52 1:27.069
53 1:26.940
54 1:26.852
55 1:26.841
56 1:26.703
57 1:26.814
58 1:26.731
59 1:26.818
60 1:26.740
61 1:26.864
62 1:26.912
63 1:27.119
64 1:27.312
65 1:28.116