TLDR - Weight training is the wrong activity for improving both your sustained speed (26-30kph) and your 1-minute speed (40kph). Read below to understand why.
There are three main types of skeletal muscles: slow twitch (type 1), fast oxidative (type 2 a) and fast glycolytic (type 2 b). Your make up of the three types determined primarily by genetics (with some interchangeability between type 2b and type 2a). These muscles differ by the metabolic pathway used to power contraction, as well as contraction speed and strength and ability to perform repeated contractions.
Slow twitch (type 1) are weak and slow but are what power sustained efforts. These muscles generate power through aerobic glycolysis (i.e., "burning" oxygen). While weaker total force output these muscles have high endurance (due to the use of oxygen). If you are doing any sustained activity over 1 minute in duration you are primarily using these muscles. If you are a professional endurance cyclist you were likely gifted by genetics with a higher proportion of type 1 fibers than the regular joe. (Aside from doping, it really is easier for them.)
If you are sprinting (efforts under 1 minute - like your 40kph efforts) you will be primarily using type 2a. When you are mashing the pedals hard in a maximal effort (under 10 sec) you are using type 2b fibers. Type 2a uses a mix of anaerobic and aerobic pathways while type 2b only use anaerobic pathways.
What weight training actually does...
That leaves us with the question of what will happen when we weight train and gained 10 kilos of muscles (hypertrophy). Muscle hypertrophy occurs primarily through chronic anaerobic, high-intensity resistance activity. This means you will be developing type 2b primarily along with some type 2a (actually changing your type 2a behave more like type 2b) and a bit of type 1 (the actual splits depends on type of resistance training).
Your ability to accelerate quickly would likely improve the most. Your 40 kph (type 2a powered) could increase a bit. Your sustained 26-30 (kph) might increase a little bit, if you kept up aerobic training as well.
The specificity principle
This gets us to the specificity principle. The training needs to be specific to the task. This comes in terms of strengthening the right muscle for the action (circular pedal motion), building up connective and support tissue (strong core is important - you can't fire a cannon from a canoe! - although...) and muscle recruitment pattern (how your brain activates individual muscle fibers).
For cycling, the circular motion is un-natural. As such we need to train your muscles with the activity at hand.
Improving sustained efforts
To improve you 26-30 kph sustained speed you would need to stay away from the weight room as we need to train type 1 muscle fibers. We can't make these muscles much bigger per say, but more efficient.
- This means improving muscle recruitment. (ask stack exchange for pedalling exercises)
- This means improving aerobic metabolism efficiency. (Long Slow Days - LSD)
- This means improving the lactic acid threshold. (Aerobic threshold efforts)
Most when they go on endurance rides they laugh at (1), then ride somewhere between (2) and (3); doing what we commonly refer to as junk miles. You will need to specifically train these three components individually to get the biggest improvements your sustained speed. A coach can be highly beneficial here.
Improving 1 minute efforts
Here we want to train our type 2a fibers and perhaps "recuit" some type 2b to be like, or function more like, like type 2a (this is a bit of a disputed topic as to the mechanism). They gym may help a bit but something like interval training (repeated hard efforts, e.g., 2 min on 1 minute off, repeated 8 times) will do wonders. Interval training is generally hard to do and hard to get right, a coach is recommended.