5 points of difference in reasearch... but you have 4 points to add more than level 26.... so the difference should not be so big.
With level 30, it was clear for me that car development should be different than now...but probably, it was not clear for all players..
The current issue is they get a lot less research than they should. Here is an example with a typical mid-late season research gap of 60 points.
L25 (27.5% with CD)
60 * 0.275 = 16.5 DP
17 DP gain + 25 DP = 42 DP
L30 (11% with CD)
60 * 0.11 = 6.6 DP
7 DP gain + 30 DP = 37 DP
This 5 DP swing in one race is as great as the L30 can ever hope for (same research- needing a miniscule gap- 25 vs 30 generated) and often late season research gaps will be larger than this, hitting 70 or 80 DP, leading to swings of up to 10 DP per race.
While the L30 will have a slight advantage of 2-3DP per race early season, the lower level accounts effortlessly overhaul this and are uncatchable by season end - the level 30 research is too weak to make saving DP viable as a tactic to mitigate this.