Problem FTB Infinity Lite 1.10 - Too many hostile entities and leaking world

[Schattennarr]

• What OS are you running?
  Debian 7 64 bit
  
  
• Are you using minecraft hosting provider or a dedicated/local server not designed for minecraft?
  V-Server, but usable for Minecraft
  
  
• What version of FTB are you using? (Dont just say latest tell us the version)
  FTB Infinity Lite v1.8.0
  
  
• Did you add any mods to the server pack?
  Decocraft 2.4.2
  Mr. Crayfishs Furniture Mod 4.1.2
  
  
• What are your server specs?
  SSD
  16 GB RAM
  Intel Xeon X5670 @2,93 GHz (4 Cores)
  
  Startup parameters:
  export MIN_RAM="12288M" # -Xms
  export MAX_RAM="14336M" # -Xmx
  export JAVA_PARAMETERS="-XX:+AggressiveOpts -XX:+UseG1GC -XX:+DisableExplicitGC -XX:MaxGCPauseMillis=50 -XX:SoftRefLRUPolicyMSPerMB=10000 -XXarallelGCThreads=3 -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M -XX:+UseStringDeduplication"
  
  
• A good detailed paragraph about the problem

First, this server is a clan server. I know the most of the players for some time and I regulary do searches for any technical things causing laggs. Most of the players are vanilly-ish-Players anyways.

So bascially I have 2 Problems:
1. After visiting the Nether and leaving it, log states leaks:

[10:01:11] [Server thread/DEBUG] [FML/]: Gathering id map for writing to world save world
[10:01:16] [Server thread/DEBUG] [FML/]: The world 7e3805e2 (world) may have leaked: seen 6040 times.

[10:01:16] [Server thread/DEBUG] [FML/]: The world be63220 (world) may have leaked: seen 6790 times.

[10:01:26] [Server thread/DEBUG] [FML/]: The world 732025b3 (world) may have leaked: seen 5850 times.

[10:01:26] [Server thread/DEBUG] [FML/]: The world 7168b4dc (world) may have leaked: seen 7385 times.

[10:01:26] [Server thread/DEBUG] [FML/]: The world 59a15bbd (world) may have leaked: seen 5230 times.

[10:01:26] [Server thread/DEBUG] [FML/]: The world 525d34e6 (world) may have leaked: seen 7260 times.

[10:01:36] [Server thread/DEBUG] [FML/]: The world 192e8cf4 (world) may have leaked: seen 7775 times.

[10:01:36] [Server thread/DEBUG] [FML/]: The world 3635b94d (world) may have leaked: seen 7405 times.

[10:01:46] [Server thread/DEBUG] [FML/]: The world 521ce84 (world) may have leaked: seen 7095 times.

[10:01:46] [Server thread/DEBUG] [FML/]: The world 561f56b7 (world) may have leaked: seen 6100 times.

[10:01:46] [Server thread/DEBUG] [FML/]: The world 7c7af531 (world) may have leaked: seen 7835 times.

[10:01:46] [Server thread/DEBUG] [FML/]: The world 39d32a1f (world) may have leaked: seen 5935 times.

The leaks add up everytime somebody enters the Nether - regardless of position. After using a search engine, I learned that it's most likely an entity or block which doesn't unload. I'm not sure if this adds additional heavy load to my server and I'm grateful for any suggestions solving this, since I have no possibilities of forceful unloading a dimension.
I tried to fix it with chunkloading the portal, but the issue remained.

2. Entity Laggs
Yesterday we had a peak of 9 players playing simultaneously, resulting in some severe laggs. After setting the difficulty to peaceful, the issue was resolved.
Can you somehow restrict how many hostile mobs are on the Server or how should I deal with it?

Outside of these two Problems: Is there a possibility by now for adding bukkit plugins to the server? I couldn't find anything for 1.10. (I hate myself for letting the clan choose the modpack. 1.7.10 would be so much easier.)