Well, first, im not sure what you mean by losing functionality.. All of the functions that are available in DarkBasic are available in the DarkGDK under c++(as far as im aware), there are also a number of undocumented functions defined in the headers, you can read through them to uncover those functions....
Sparky's collision DLL works very well with DarkGDK, i have used it myself, and all of the DarkBasic plugins that you have mentioned are mostly also availalbe to DarkGDK users. If you vist your Order History where you originally downloaded your plugins from, you should be able to just download the GDK version aswell straight up(no extra cost provided that you have already purchased the product, you can download DBP and GDK versions of plugins that you have purchased).
The core engine, im not sure on that one, i believe it is the current DarkBasic Pro engine under the hood, but perhaps one of TGC devs would be able to better answer that one.
As for the advantages of progamming in C++ over Darkbasic, they are almost too numerous to mention, compeltely aside from the DarkBasic/GDK side of things, C++ will allow you to plug your programs directly into DirectX, MFC(not express editions) etc, C++ is an object oriented language, allowing you to structure your programs much more efficiently than a procedural language such as DarkBasic. What i mean, is that the "Pros" are that C++ is a superior development environment in almost every way compared to darkbasic(my opinion, others may think differently). Programs compile faster and run faster. There is also the issue that you raised in you post in regards to coming out of school/college.. Knowing how to code in C++, would allow alot more freedom in regards to job prospects and entry into the industry as opposed to DarkBasic, and not just in the gaming industry...
The disadvantes.... well, there is one big one... Learning to code properly in C++. It can be a daunting and time consuming task, once learnt however, the degree of control you get over your apps in C++ is second to none and much greater than that afforded by DarkBasic, also, the internet contains a massive amount of resources dedicated to teaching people the fundamentals of C++ and beyond..
For C++, I would recommend to everybody as a first read :
Ivor Horton's Beginning Visual C++ 2008
Product Details
* ISBN: 0470225904
* ISBN-13: 9780470225905
* Format: Paperback, 1356pp
* Publisher: Wrox Press, Inc.
* Pub. Date: March 2008
I have found this book to be an invaluable resource. It details the VS C++ 2008 IDE, and although based around the Commercial Editions, it talks about everything from the very basic "hello world" console programs, through to fully developed applications, ANSI/ISO, CLR, .NET, MFC... everything.. Just google the title, it shouldnt be too hard to find a copy(got mine thru amazon).
This book is just my personal perference, there are, as i have said, an almost unlimited resource of C++ tutorials/lessons on the web, aimed at everybody from the total novice to total professional.. again, google away
Hope I have been of some help.. Good Luck
If it ain't broke.... DONT FIX IT !!!